What idea does Piggy have that Ralph laughs at?

Piggy wants to make a sundial.

"I've been thinking," he said, "about a clock. We could make a sundial. We could put a stick in the sand, and then--"

The effort to express the mathematical processes involved was too great. He made a few passes instead.

"And an airplane, and a TV set," said Ralph sourly, "and a steam engine."

Piggy shook his head.

"You have to have a lot of metal things for that," he said, "and we haven't got no metal.

But we got a stick."

Ralph turned and smiled involuntarily. Piggy was a bore; his fat, his ass-mar and his matter-of-fact ideas were dull, but there.