What is critical m in a nuclear reaction, please explain as if you were talking to a dumb monkey?

I know that the two subcritical mes in a uranium nuclear bomb collide and then become a critical m and a chain reactional is optional. What I don't understand, is how two objects, of the same density, can become an object that has both its size and is denser, which is what they say on every page I visited. Or is it something I don't quite get? As I said earlier, although I'm 13 with a 131 IQ, please explain like you would to a 5-year old. Thanks a bunch.