Bubbles are a pocket of air surrounded by a thin coating of something else. Bubbles are usually made from blowing air into soapy water. They are round, and float through the air. There are pretty colours on the surface of the bubble.
A cocoon is a place a caterpillar makes for itself. The caterpillar goes to sleep inside it for some time and then comes out as a butterfly.
Birds build nests to lay their eggs in. Nests are usually made from sticks and lined with feathers and anything soft that the bird can find, such as threads from clothes and even cobwebs. Some birds make nests out of mud.
A shell is the hard outside coating found on many animals. Snails have shells that form a spiral that was built up as the snail grew bigger. crabs and turtles have a hard shell that perfectly fits the animal inside. Most animals that have shells can hide inside them if they are scared.
Spiders spin webs between twigs (or anything) to catch insects in. Webs are sticky, so insects that fly into the web get stuck. The spider sits on the web and feels for the vibrations of a struggling insect with its feet, and then comes over to wrap it up in more webbing and eat it.