Other forms: pencils; penciled; pencilling; penciling
If you want to write something you can easily erase, write it with a pencil (instead of with a pen). When you color in the bubbles on a standardized test, a #2 pencil is the way to go.
Unlike pens, which use ink, pencils have a pointed cylinder of graphite that makes marks on paper. Most pencils are wood with a thin center of graphite, although mechanical pencils are made of metal or plastic and look more like pens. You might describe an artwork as a pencil drawing, or ask if you can pencil some notes in the margins of your friend's copy of "Moby Dick." When you "pencil something in," you mark it tentatively on your calendar, knowing you might have to erase it later if your plans change.