They usually come in children's craft kits or magazines in a strip of about 5 colours complete with a token paint brush. They are impossible to open without using your teeth / a knife / breaking all your nails. They contain a trivial amount of paint that certainly isn't enough for the project they are intended for. By the time you've managed to open each one your child has completely lost interest and moved onto play with something else.
They are up there with those toys that come with 5000 plastic ties to attach them to the box.