The Grapes of Generosity are magical artifacts which appear when a Fraggle needs to learn how to be generous. They're so delicious that a Fraggle will want to eat them all himself, but if he doesn't share them, he eventually becomes weightless so that everyone can see how selfish he is as punishment. If he is generous again, his weight comes back.
They appeared in a third season episode of Fraggle Rock called "The Grapes of Generosity."