Silk worms are actually a type of caterpillar that spins silk cocoons around themselves after they turn approximately 1 month old. Whenever they are finally ready to start spinning their cocoon, they will stop eating any food and then they will turn yellowish. At this point, it will then take the silk worm approximately 3 days to spin their cocoon around themselves. If their threads are disturbed by anything, the silk worm will have to start all over again spinning a new cocoon.