A common dessert in my family is a mix of crushed biscuit, chopped walnuts, coconut, cocoa, and condensed milk. It's mixed, flattened into a square dish and placed in the fridge for a couple of hours, after which it is quite solid.
I have always wondered what the mechanism was which caused it to go so solid, since each ingredient on their own doesn't solidify in the fridge. Sometimes you see melted butter in a recipe for this purpose but there isn't any in this one. Why does it go hard in the fridge?