Another education cock-up

I received this from my friend John today : A primary school class started a class project to make planters to take home to their parents. They wanted to have a plant in it that was easy to take care of, so they decided to use cactus plants. The children were given green-ware pottery planters in the shape of clowns which they painted with glaze. The clown planters were professionally fired at a class outing so they could see the process. It was great fun! They planted cactus seeds in the finished planters and they grew nicely, but unfortunately, the children were not allowed to take them home. See if you can guess why.