The little bumps on the ceiling!
The texture is called "popcorn". It is very light, and made of puffed foam or plastic. It is sprayed onto the ceiling with a special machine. There are other textures that are made with something similar to drywall compound. It can be rolled on with a special roller that leaves random lines bumps of excess drywall compound that dries and can be painted.
"I was jumping on my bed, but I jumped too high and hit my head on the ceiling and some of the trivecorns came off."
"Check the ceiling, God's got nice trivecorns."
"The trivecorns must be respected."