Silkworms are very hungry. They eat mulberry leaves day and night. Silkworm eggs take about ten days to hatch. When the colour of their heads turns darker, it means that it is time for them to shed their skins. After they shed four times, their bodies turn slightly yellow and their skin becomes tighter, which means they are going to become a pupa. While they are a pupa, they turn into moths. Before becoming a pupa, the silkworm wraps itself in a cocoon, to help protect itself. The silkworm is called a silkworm because it spins its cocoon from raw silk that it makes in its mouth. Many other larva produce cocoons, but only a few silkworms are used to make silk.