What's yellow and writes? A ball-point banana.
A physics teacher writes a question on a board:

"A 40 kg child that 100 cm tall is holding a parent's arms swinging them 0.5 revolutions a second. If the parent let go of the child after 2 seconds, where will the child end up?"

A few moments later, the teacher then comes over and reads a student's answer:

"In a foster home."
I found a pen that writes underwater.
It writes other words too.
How do two programmers make money?
One writes viruses, the other anti-viruses.