Stern Lesson is a common Instant card from The Brothers' War (BRO), collector number 64. Current market price: $0.33 (foil $0.17), TCGplayer market prices via Scryfall.
Draw two cards, then discard a card. Create a tapped Powerstone token. (It's an artifact with "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell.")