In my classroom aquarium, one of the corydoras catfish was laying eggs. She would stick them to the glass in little clusters. Shortly after she laid them, the other fish would eat them.
At first, the kids were amazed at the egg-laying. Then they got upset at the egg-eating. They stood by the glass and tried to scare the other fish away from eating the eggs. It would only sort of work, then the fish would eat the eggs, because it was fresh and tasty food.
It was a lesson in life. And a terrible disruption to the class day.
(I forgot to take a picture)