Halo3What is the deal with skulls? How do you "claim" a skull? I have found one and I pick it up, then what?

Halo3 - I found the 1st skull in the beginning of the game, (on the ledge near the water fall) I can pick it up, but then I can only hold onto the skull, no weapon, no grenades? I just dont understand. Please be remedial, I am an idiot!