If iI remember right, they build a bubble nest. Try this page for lots of betta breeding info:
What JI'm says is right, Bettas bui;d bubble nests.
yup. The males make the bubble nests. It is a sign of them being in great condition, and they also start it when they see a female betta. Then, if your breading, the female judges it to see if it is worthy of her having eggs to go in it.
Yea, Bettas build bubble nests.. either as an breeding urge or sometimes weather can prompt them and sometimes they just get in the mood it seems.
I think it's funny. It's almost like they have a chew of tobacco in their mouth and they have to spit or something.