Malk is a drink which is served with the lunches at Springfield Elementary School instead of milk. It has "vitamin R" and doesn't seem to have any nutritional value.
Bart thought that the school served milk, but when he cracked his knuckles, they snapped painfully. Bart was shocked about the brittleness of his bones because he had always drunk plenty of milk. However, when he looked closer at the carton, he realized it was written "MALK" instead of "MILK."