How can we jump when the ground does no work on us? This may seem like a silly question, but how can we jump when the ground does no work on us?
When we jump off the ground, the ground does not move. This means the force from the ground on us does zero total work. Since the force does no work, we cannot gain any kinetic energy so we can't jump.
 A: The ground actually does move, by an infinitesimal amount (c.f. conservation of momentum). But I imagine that's not what you're asking.
The reason you gain energy is because your body converts energy it has already stored (in the form of Adenosine Triphosphate) into mechanical energy that powers your muscles. In other words, you can jump because you are a living being with energy stored, capable of doing work without external input. Put a block of wood on the floor, and you will never see it spontaneously "jump".
A: Your muscles are doing work that becomes your kinetic energy associated with the initial speed when jumping. When you jump, some form of chemical energy becomes kinetic energy of you and earth. However, since earth is super heavy, the kinetic energy it receives is essentially zero while you receive essentially all the kinetic energy. The conservation of momentum equation and conversation of energy (KE1+KE2=ChemicalEnergy) equation (with given masses and ChemicalEnergy) will determins the two variables that are the initial speed of you jumping and earth recoil, which is essentially zero when the mass of earth approaches infinity.
