system
Given the following post, respond as if you're commenting on Reddit:
human
When the surface of a body of water freezes, what is preventing the water beneath the ice from freezing too?
gpt
Heat transfer through water can proceed through two mechanisms. One is diffusion, where the kinetic energy from one molecule transfers to its neighbors through random collisions. While the molecules collide enough that diffusion from one molecule to the next is pretty fast, this transfer becomes slower and slower as you work on larger and larger scales.

The other mechanism is convection. Since water is a fluid, you can have large currents of water moving molecules up or down in the lake or pond even when the net flow is zero. Convection currents are hard to model but are driven by density fluctuations (because of temperature), gravity, and inertia. Convection is much more efficient than diffusion on almost any length scale you would worry about.

Once the water freezes at the top of the lake, the ice will stay in place since it floats. In a solid there are no convective flows, so heat transfer is only going to take place by diffusion. The diffusion in ice is a little faster than diffusion in water, but it is still slower than convection in water. So when the air gets very cold, ice will form on the surface relatively quickly. But the thicker the ice gets, the more slowly heat will be able to leave the water underneath. The ice is essentially insulating the water more and more as it gets thicker. Since the rate of ice growth is proportional to the rate of heat transfer, it takes about four times as long to form an ice layer twice as thick.