In Pharaoh's second dream, seven thin heads of grain swallowed up what?

Answer: Seven plump heads of grain

Seven plump heads of grain grew on one stalk; then seven thin, scorched heads swallowed them up (KJV: "ears of corn").

“but he fell back asleep and dreamed a second time: Seven heads of grain, plump and ripe, came up on one stalk. After them, seven other heads of grain sprouted, thin and scorched by the east wind. And the thin heads of grain swallowed up the seven plump, ripe ones. Then Pharaoh awoke and realized it was a dream.”— Genesis 41:5-7 (BSB)