in 4 season countries, the temperatire may drop till below 0°c during the peak of winter. why does the surface of the river or lake freeze?

Explained below

Explanation:

Normally, when the temperature of water gets colder than 4° C, it starts expanding and will become less dense the colder it becomes. As the temperature gets closer to freezing point, the colder water will float to the top and the warmer water sinks to the bottom. This is mainly because of how the density of water behaves with falling temperatures.

Thus, the lake/river water will freeze from top to down as a a result of that phenomenon of colder water floating to the top and the warmer water sinking to the bottom.

gold is my favorite solid.