Ice is slippery because it is lubricated by melt water as viscous as oil, according to experiments that teamed a tuning fork with a powerful microscope.

In the conventional explanation for ice’s slickness, objects melt the ice as they move across it, forming a thin layer of water on which those objects glide. But water is typically a poor lubricant because of its relatively low viscosity.

A team led by Alessandro Siria and Lyderic Bocquet at the École Normale Supérieure in Paris examined this melt water using a new ultra-high-resolution microscope equipped with a tuning fork. When the fork’s tip was made to vibrate, it dragged a glass bead back and forth across a layer of ice, allowing researchers to measure the friction generated by the bead’s passage. The bead also moved into and out of the ice, providing data on the viscosity of the meltwater film between the bead and the ice.

Surprisingly, the authors found that this melt water is up to 100 times more viscous than normal water, making it an excellent lubricant.