What should I use to mark the eggs of my setting hen?

[SnuggleBear]

What should I use to mark the eggs of a setting hen so I will know which eggs were laid that day and which she is sitting on?

I think I had a hen go sit yesterday and this is my first time.

Thank you

 

[Egghead_Jr]

I mark my eggs with a pencil. It's non toxic and unobtrusive to the shell being permeable.