What did Gideon lay on the threshing floor to test God's promise?

Answer: A fleece of wool

Gideon asked for dew only on the fleece, then the next night dew only on the ground. God did both.

“then behold, I will place a fleece of wool on the threshing floor. If there is dew only on the fleece and all the ground is dry, then I will know that You are going to save Israel by my hand, as You have said.””— Judges 6:37 (BSB)