Have you ever watched a 20-kilogram slab of granite glide down a sheet of ice and bend around a guard as if it knew where it was going? Welcome, dear reader — that bend is one of the best physics puzzles hiding in plain sight, and we are going to solve as much of it as science currently allows.
The Milano-Cortina 2026 Winter Games have come and gone, but the question they left on the ice has not aged a day. Stay with us to the end and you will watch the next curling match the way an ice physicist does.
Updated July 2026: reframed after the close of the Milano-Cortina 2026 Games, with a corrected account of the curl mechanism, a corrected sweeping-workload figure, and a cleaned-up source list.
TL;DR — The Direct Answer. A curling stone curls mainly by steering along microscopic scratches that its rough running band carves into the pebbled ice, aided by friction differences across the band. Sweeping melts the tops of the pebbles, lowering friction so the stone runs farther and straighter. The complete mechanism is still an open research question.
Why Does a Curling Stone Curl?
The stone steers itself along scratches it has just made. The rough granite of the running band carves microscopic grooves into the pebbled ice, and a fraction of a second later the trailing edge of the band crosses those grooves and follows them. That, in one sentence, is the leading explanation for why a curling stone curls, and it took physicists the better part of a century to reach it.
Here is what makes the puzzle so stubborn. Slide an overturned drinking glass down a table with a clockwise spin and it drifts left. Give a curling stone the same clockwise spin and it drifts right. Ordinary sliding friction predicts the glass; the stone breaks the rule, which is exactly what a Swedish team at Uppsala University set out to explain in 2013.
Their answer was scratch guiding. When Harald Nyberg and his colleagues pre-scratched the ice in chosen directions, stones dutifully followed the grooves, and even stones released without any rotation could be steered this way. The granite writes its path, then reads it.
Rotation, meanwhile, plays a smaller role than you would guess. Two or three turns over the whole sheet. Far too slow for any aerodynamic effect of the kind that bends a curveball.
More recent measurements, discussed in detail by curling researcher Glenn Paulley, suggest the stone passes through three distinct friction regimes on its roughly 25-second trip:
- Wet friction at high speed, when frictional heating keeps a thin lubricating water film under the running band even though the ice sits near minus four degrees Celsius.
- Mixed friction at moderate speed, past the break point, as the band begins to bite into the pebbles and scratch them.
- Dry friction in the final meters, where no film survives and the band rides across, and along, the scratches it made moments earlier.
A 2022 study in Nature Scientific Reports, led by Jiro Murata at Rikkyo University, used precision kinematic measurements to argue that the stone also pivots on discrete pebble tops, a point-like friction effect that amplifies the curl in the slow final phase. The models still disagree on the details, and ice makers exploit that sensitivity every day: a small change in pebble size or ice temperature visibly changes how much a sheet curls.
What Makes Ailsa Craig Granite the Only Choice?
Two properties settle it: hardness that shrugs off decades of collisions, and water absorption close to zero. Nearly every stone that slid across the Cortina ice was quarried on Ailsa Craig, a small uninhabited island in the Firth of Clyde off the west coast of Scotland.
The rock formed about 60 million years ago, as the landmass connecting North America, Greenland and Europe tore apart and magma cooled underground into what geologists call a microgranite. Its quartz and feldspar crystals interlock so tightly that Phil Stone of the British Geological Survey describes the material as “a bit of a devil to break”. Godfrey Fitton, professor of igneous petrology at the University of Edinburgh, compares the crystal structure to a mesh.
The running band, the narrow ring of granite that actually touches the ice, is cut from the finest grade, Blue Hone. Water expands by roughly nine percent when it freezes, so a stone that absorbed even a little moisture would crack apart under constant freeze-thaw cycling. Blue Hone lets almost nothing in. That is engineering necessity rather than tradition.
Friction and granite pull off quiet miracles away from the rink too. The pairing is part of what holds Kummakivi, Finland’s famous balancing boulder, on its narrow rock perch.
How Does Sweeping Steer the Stone?
Sweeping steers with heat. Fast, hard brush strokes just ahead of the stone momentarily melt the tops of the ice pebbles, and the resulting molecularly thin water film lowers the friction under the running band.
Two things follow. The stone travels farther, sometimes the full difference between a stone that counts and one that dies in front of the house. And it runs straighter, since lower friction also shrinks the asymmetries that produce curl, which gives the team a way to trade curl for reach in real time.
A correction is due here. An earlier version of this article stated that twenty seconds of hard sweeping produces 600 to 1,600 kilojoules of work. That figure is off by roughly a factor of a thousand: it would demand the sustained power output of a small car engine, and no human body delivers that. We have removed it, and the honest numbers are striking enough on their own.
Sports-science measurements put average heart rates during intense sweeping around 170 beats per minute, with peaks approaching 200, in the range of 90 to 94 percent of heart-rate reserve. Over a full game a front-end player may cover about 1.7 kilometers, much of it crouched and pushing down hard at sprint intensity.
Then they stand up and chat about strategy as if nothing happened.
What Does Curling Ask of an Athlete’s Body?
It asks for two engines and a fast gearbox between them. The anaerobic system powers the 10-to-20-second sweeping bursts, the same energy pathway a sprinter fires off the blocks. The aerobic system then clears the metabolic debris and restocks the muscles before the next stone.
The hard part is the switch. Within about two minutes of near-maximum exertion, a curler may need to deliver a draw weighted to the centimeter, and nobody throws a precise stone with a hammering heart and shaking forearms. Research on heart-rate recovery in curlers documents exactly this toggle, from roughly 90 percent of heart-rate reserve back down to fine motor control, over and over across a match.
That is why elite teams train interval recovery as deliberately as they train the delivery itself. The physiological profile fits no other winter sport neatly.
Why Do Curlers Call Their Own Fouls?
They do it because the sport’s own constitution tells them to, and the culture enforces it harder than any referee could. The World Curling Federation codifies the ethos as the Spirit of Curling, and it states plainly that “a true curler would prefer to lose rather than win unfairly”.
Touch a moving stone with your brush and you announce it yourself, even in an Olympic final. No trash talk, and no celebrations aimed at the other team. Part of this is practical, since the game has too many subtle contact rules for officials to police every one. Part of it is heritage, carried forward from small Scottish communities where your word settled the match. Either way, it survives intact at the highest level of the sport.
The Verdict
So the curl comes from scratches in the ice, friction that changes regime three times in 25 seconds, granite from one Scottish island, and athletes who flip between sprint power and surgical touch on demand. Layer on a written code of honor and you have a sport that rewards close watching long after the medals are packed away.
We like curling for the same reason we like any good open problem: the everyday physics is not finished. Competing models still argue over the last word on the curl, the way bigger questions in physics stay open, like whether we can ever catch a graviton in the act. Watching careful people close those gaps, one measurement at a time, is the whole show.
This article was written specifically for you by FreeAstroScience.com, where complex scientific principles are explained in simple terms. Come back soon, and never turn off your mind, because the sleep of reason breeds monsters.
Gerd Dani
President, FreeAstroScience — Science and Cultural Group
Frequently Asked Questions
Why does a curling stone curl in the direction of its rotation?
A curling stone curls mainly by steering along microscopic scratches that its rough running band carves into the pebbled ice. Friction differences across the band and pivoting on individual pebbles add to the effect. The spin sets the direction of the scratches, so the stone follows the direction of its rotation.
Why do curlers sweep the ice in front of the stone?
Sweeping warms the tops of the ice pebbles and creates a microscopic water layer that lowers friction. Lower friction lets the stone travel farther and reduces the sideways drag difference that causes curl, so a swept stone runs straighter. Sweepers adjust effort in real time to control both distance and line.
Where do Olympic curling stones come from?
Nearly all Olympic curling stones are made from granite quarried on Ailsa Craig, a small island off the west coast of Scotland. Its Blue Hone microgranite has a dense interlocking crystal structure that absorbs almost no water, so freeze and thaw cycles cannot crack the running band that touches the ice.
How many times does a curling stone rotate during a shot?
A curling stone typically rotates only two to three times during its full journey down the sheet, which takes about 25 seconds. That slow spin rules out aerodynamic effects like those on a curveball. The curve comes instead from friction differences and from scratches the stone leaves in the pebbled ice.
What is the Spirit of Curling?
The Spirit of Curling is a written code of conduct upheld by the World Curling Federation. Players call their own rule violations and avoid distracting opponents, accepting that winning unfairly is worse than losing. The code makes athletes, not referees, the primary guardians of fair play at every level.
Sources
- Focus.it — “Milano-Cortina 2026: la scienza segreta dietro al fascino del curling alle Olimpiadi invernali” (February 5, 2026).
- Olympics.com — “How does the combination of pebbled ice and stone shape curling?” (February 3, 2026) — olympics.com
- Glenn Paulley — “We are getting closer to understanding the physics of curling” (March 2025) — glennpaulley.ca
- Murata, J. — “Study of curling mechanism by precision kinematic measurements”, Nature Scientific Reports (2022) — DOI: 10.1038/s41598-022-19303-4 — nature.com
- Phys.org — “The mechanism that puts the curl in the curling stone revealed” (May 2013), on the Uppsala University scratch-guiding experiments by Harald Nyberg and colleagues — phys.org
- Rikkyo University — “Solving the century-old mystery of why curling stones curl” (September 2022) — rikkyo.ac.jp
- Science Friday — “Why Olympic Curling Stones Come From This Scottish Island” — sciencefriday.com
- National Museums Scotland — “The Roaring Game: Scotland’s curling stone island” — nms.ac.uk
- World Curling Federation — “Spirit of Curling” — capitalcurlingclub.org
- Bradley, J. L. — “The Sports Science of Curling: A Practical Review”, Journal of Sports Science and Medicine, 8(4), 495–500 (2009).
- The Curling Club — “Why Do They Sweep? The Physics of Curling Explained” (December 2025) — thecurlingclub.com
- CurlTech — “Ice and Rocks” (January 2026) — curltech.com
