What kind of energy is stored when you squeeze a mattress?

When you squeeze a mattress, and you release it, you can see that return to the original shape. So, when you are squeezing it, the mattress "want" to return to the original shape. The mattress does it by storing elastic energy, and when you stop squeezing it, uses the elastic energy to return to the original shape.

so the energy stored is elastic energy.

a straight line sloped

measure their densities same density, same sub