Okay kiddo, have you ever seen a balloon being inflated? That's kind of what happens with a pneumatic lifting bag. It's like a balloon, but it's made out of strong material that can hold a lot of air.
When we want to lift something really heavy, we put the pneumatic lifting bag underneath it. Then we use a machine to blow air into the bag - just like blowing up a balloon! When the bag fills up with air, it gets bigger and bigger until it's big enough to lift the heavy object off the ground.
The really cool thing about the pneumatic lifting bag is that it's really strong, so it can hold up even the heaviest objects. And when we're done lifting, we just let the air out of the bag and it deflates back to its original size so we can use it again.
So, it's like a balloon that lifts things and is super duper strong!