When you pressed the balloon, the shape of the gas inside changed and got compressed.Gas has no definite shape, so it takes the shape of its container — in this case, the balloon. When you press it, you reduce the space inside, so the gas particles get squeezed closer together, changing the shape but not the amount of gas.In short: The gas adjusted to the pressure and changed shape along with the balloon, showing that gas is compressible and takes the shape of its container.