A frozen river is like a giant ice cube made out of the water in the river. Just like when we make ice cubes in the freezer, when it gets really cold outside, the water in the river freezes and turns into a hard, slippery surface that we can walk on. It's like a big, cold playground!
But frozen rivers can be dangerous too. Sometimes, the ice can be thin and break when you walk or skate on it. It's also important to remember that even if the surface looks solid, there might be areas that are weaker and more likely to break. So it's always important to be careful and stay safe when playing on a frozen river.