How to mark eggs for broody?

[MeatKing]

I was told to mark eggs so I know witch ones are "the ones she's been sitting" not just random eggs... so with what do you mark them? I thought about a sharpie but then am worried that mabe ink will go through egg and hurt development somehow. Thank-you for your suggestions everyone

I'm gonna put them under her today

 

[bakerjw]

I used a pencil. It doesn't rub off and is pretty safe.
Avoid markers or anything that might contaminate the egg and possibly affect the development of the chick.

ETA.
BTW. I put some range eggs under a broody and she hatched out 7 of 11 over the weekend. They're late in the year but they still hatched and are happy and lively lil fuzzy butts.