But the truth was the drain had made him very slimy, and it was necessary for him to take a bath and sprinkle himself with a bit of his mother's violet water before he felt himself again.
The back of Harry's robes was damp. One of the jars behind him had broken when he fell against it; the pickled slimy thing within was swirling in its draining potion.
Pond scum is the slang name for microalgae - green plant organisms, such as spirulina, which grow in water - like ponds and look a bit slimy or scummy before being dried.
They smash some of said pumpkins on said steep hill, and then they slide down the slimy pumpkins inards on sleds, boxes, plastic pools, whatever they can find really.