I have two male rabbits and they have a large water bottle with the little bead that attaches to the outside of their cage. Every time I fill it up within a half an hour they've spilled all the water. I'm pretty sure they lean up against the bead until all the water's gone. Does anyone know how to make them stop?

Also, it is a wire cage and the water bottle's on the outside. The only part inside the cage is the little metal neck.