Why Do Butterflies Drink Water?

It was a Sunday yesterday, and out of the curiosity to test the macro feature on my camera lens I figured I'd pay a visit to the Kuala Lumpur Butterfly Park. The park is located near the Kuala Lumpur Bird Park, and the entrance fee is only RM 10 for Malaysian, but RM 20 for foreign visitors.

While shooting pictures of the critters I noticed something strange: the butterflies intermittently danced around and landed on the wet floor.

Are they drinking water?
But why?