Okay, little buddy! Do you know what a dumper is? It's a big truck that helps carry dirt, sand, or rocks. When we say 'dumpers don't have a roof', it means that these trucks are open at the top. Imagine if you had a toy truck that didn't have a cover on top, like a little car. This way, they can easily dump what they are carrying out when they lift the back part. Just like when you tilt your toy truck and all the tiny blocks fall out. So, no roof means the stuff can come out easily!