Today I say some air bubbles coming from my aquarium filter, which isn't normal, so it was a bit of a surprise. While I was watching, it happened a second time. Hey! That's not supposed to happen... Now where could this air be coming from?

I looked very hard at the filter, and finally saw... one of the African clawed frogs, who had been sitting underneath the filter and belching  :-)
He went up for air immediately after that, so he was OK, it was just a silly coincidence that his froggy burps went into the filter...