Crystal

Crystals are the main food of your monsters. You grow them at a Crystal Farm. You level your monsters by feeding your monster three times. To level from level 1 to 2 10 crystals at a time, and the number of crystals per feeding doubles every level, requiring 20 crystals at level 2, and 40 at level 3.

Crystal Farm
You start with a single Crystal Farm, and can buy more. You can also buy a Gold Crystal Farm with gold, which produces 3x the normal crystal. Your second farm costs 2000, and third costs 4000.