Thinking Cap is a common Artifact — Equipment card from Murders at Karlov Manor (MKM), collector number 257. Current market price: $0.09 (foil $0.22), TCGplayer market prices via Scryfall.
Equipped creature gets +1/+2. Equip Detective {1} Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)