My male Betta fish just made a bubble nest, my girl and boy Betta fish are...

[KimBree?]

...separated in a 10 gallon tank.....? I'm not going to mate them. My question is, what is the bubble nest made out of? Where did it come from? If I was to mate them how would I know if the boy is to large for the girl? Does the girl notice he has made a bubble nest? Again they are separated in a 10 gallon tank.
He keeps standing under this nest he's made. It's kinda scaring me.

[AzureLynn]

Male bettas always make bubble nests. It's their way of showing they are happy. And used for courting/mating purposes also if a girl is in the same tank. Since you have them separated no worries.

The bubbles are something akin to spit to hold the air in the bubble.

[S H]

Good that you aren't going to let them spawn.

The next is basically bubbles of air and his spit, which helps glue the bubbles together and keep them from popping. The girl probably does not notice the nest. And he is underneath it to guard it. That nest is his 'territory'.

Male bettas will bubblenest even when they are not in the presence of a female.