The other giggles in response. The pattern is repeated until you think you will go nuts listening to this. It might go on a long time because the girls are doing something and being rewarded for doing it. A vicious circle is established where the giggling is the stimulus and also the reward. These kinds of things do tend to go on a long time and to get worse as they continue.
Children seem to be trained by our culture to do things in particular patterns.
For example, the two girls just described giggle. Two boys probably wouldn't be caught dead giggling because that's girl stuff. Boys will probably do the equivalent thing by shoving, hitting, pushing, or doing some other obnoxious thing to each other. They will tend to laugh and do the boy equivalent of giggling while they are doing it. So far, this isn't too hard to take.