Cave-in

Imagine you're playing with a big pile of sand or dirt. You make a tunnel or a hole in the pile and crawl inside. The sand or dirt is holding up the roof of your tunnel or hole. But if too much sand or dirt is taken away, the roof won't be supported anymore and it can collapse or "cave-in." This means that the sand or dirt falls down, covering you up and making it hard for you to breathe or move. A cave-in can be very dangerous, so it's important to be careful when digging or playing in piles of sand or dirt.