Pens are sticks that write, sort of like chalk but smaller and in darker colors. Ah, yes, that is paper. Just don't write on that. Look, here's a piece of paper that you can write on. Uh, no, that's a ruler.
Rulers are for measuring things.
Uh, no, that's a toothpick, it's for cleaning teeth, I don't know why it's on my desk. Look, here's a pen. Little sticks are brown. Just use this. Uh, you have to take the cap off first.