Teenagers like to try to embarrass their teachers and I had two such instances today: 'what is a transexual?' and 'is the plural of penis peni?' I consider myself as being someone who generally turns a very rosy colour at anything to do with sex, reproductive organs or even remotely related to body parts, even thumbs, but as a teacher you've got to look them straight in the eye and say:

'A transsexual is someone with both male and female reproductive organs'. Wrong. I'm such a geek I looked it up later - it's a good job I don't have to teach too much sex education because there'd be some very confused kids out there. Whilst we're on the subject, a woman can't get pregnant if you do it standing up right?

And: 'I think the plural is just penises'. Which is good enough though penes is also correct and I think sounds a little more comical. Though I'd be hard pushed to think of a situation where it'd be required. How often do people spend time discussing more than one penis? Personally it's rare I'll even discuss that many.