SALISBURY, Mass. (AP) -- Kittens doing the potty dance at a Salisbury shelter had good reason. Officials at the Merrimack River Feline Rescue Society say someone stole the shelter's entire supply of kitty litter over the weekend. Liz Pease, director of operations, says five or six 40-pound bags of wood pellets, which are used as kitty litter, stored in the shelter's unlocked shed behind the main building were taken. The bags cost about $6 each. Pease tells The Daily News of Newburyport that she suspects someone stole the wood pellets to heat their home. The wood pellets control odor better than traditional kitty litter. The roughly 60 cats in the no-kill shelter avoided accidents because volunteers started dropping off bags of kitty litter. Pease says the shed will be locked from now on.