williemon

When the water gets hot, it expands. If it gets too hot, it will boil creating steam and more pressure that will open the cap and allow water to pass out to the overflow. If it does not get too hot, but just leaks out the system while driving, like when it leaks into a cylinder, then in both examples, as the water cools, it shrinks, thus creating a vacume in the radiator, that will allow the cap to seep or flow water from the jug back into the radiator.

Bundubasher

Yep, that's the theory - just in practice it just doesn't happen with my rad!