Once upon a time, before the development of modern plant breeding, it was only in fairy tales that a young lady could ride to a grand ball in a giant pumpkin. Now the biggest pumpkins are well able to accommodate several passengers. (The notion of pumpkin transport was made real this year when a man from Massachusetts put an outboard motor on his home-grown giant and piloted it across Boston Harbor.)

How is this biologically or physically possible? At the opening of this decade, growers of giant pumpkins saw a one-ton pumpkin the way runners once saw the four-minute mile. But the ton barrier was crossed in 2012, and record pumpkins have gotten larger every year. Surely there must be some maximum pumpkin size set by the laws of physics and the biology of squashes. Can they just keep getting bigger forever?

Watch out, say some scientists. Pumpkins could, in theory, get a lot bigger.