The Grab Bag will spawn 1-4 random pickups upon touching it. They will not drop any hearts, pills, or chests, and they have an increased chance to drop batteries. Sack Head has a 33% chance to replace pickups with sacks. Shops have a chance to sell sacks for seven coins.
A Black Sack will spawn 2-3 pills, black or bone hearts, and/or bombs when collected. Only one heart can spawn from one sack, but there may be two pills or bombs.