It's cold outside right now, and the biggest river in the country has frozen over. We're talking about a minimum of 500m in width, and I've no idea how deep (but some pretty big ships can sail there).
And this got me wondering - how does a big river like that freeze over? When water freezes, it starts off small - thin membranes of ice, tiny grains, etc. But the water is constantly moving. Any paper-thin layer ice that could form would be broken apart immediately. And yet, given the right conditions, it can somehow freeze over thickly enough that a car can be safely driven over.
So how does this process happen? How can a large, moving, undulating river just freeze over without the ice breaking apart as it does so?