A zookeeper wants to add a platypus exhibit so he starts typing a letter to the zoo's animal supplier:
Dear Sirs, please send me two platypusses.
Hmmm, that didn't sound right so he rips the paper out of the typewriter, crumples it up and starts again with a new sheet of paper:
Dear Sirs, please send me two platypi.
Nope, that didn't sound right either so he rips out that sheet of paper too. He thinks hard for a few minutes, then inserts a clean sheet of paper and types:
Dear Sirs, please send me a platypus. And while you're at it, send me another one.