Diet cola

Two blondes decided to split a can of Diet cola. One blonde opened the can, and poured half the contents into her own glass, and half into her friend's glass. Before tossing the can, she stopped to read the nutritional information on the side.

"'Only one calorie per can'," she read aloud.

"Hmm," murmured the other blonde. "I wonder which glass has the calorie?"