Energy, as represented by the hearts counter, is required in order to play each level. If you want to get a free energy restoration right away without having to screw with buying more energy or waiting around for it, then:

Tip 1: Go to the date and time settings on your phone and set the time ahead by about an hour or so. Then go back to the game and you will have a full restoration.

Tip 2: The only catch when you do this, though, is that when you set the time back to normal, the energy will all be gone again.

Tip 3: If you use all of the energy and do this trick multiple times, then set the time back to normal, the previously-added time will also be added back to the energy timer, meaning it will take forever for a heart restoration.