Hey kiddo, do you remember how a balloon has air inside it? And how when we press it, it bounces back? Well, a hydropneumatic device is something like that but with water and air inside.
Imagine a container (let's call it a tank) that has water and air inside it. When we pump more water into the tank, the air that was inside the tank gets compressed due to the pressure, just like when we press a balloon.
Now, what happens when we release the water from the tank? The compressed air gets a chance to expand and it pushes the water out of the tank. This helps in creating a force to do useful work.
For example, a hydropneumatic device can be used in vehicles to control the suspension system. When the tires hit a bump on the road, the device absorbs the shock and bounces back to its original shape, giving the passengers a smoother ride.
So, in short, a hydropneumatic device is a system that uses water and air to create pressure and force to perform various tasks.