This is a test of commonsense. Complete the next sentence:

How to add lead to a mechanical pencil
Replace cassettes.
Exact instructions may vary by model, but generally, start by pulling the eraser out of the pencil, which will draw out the old cassette. Give the old cassette a shake just to make sure that it really is empty.
If so, insert a new cassette into the pencil's open chamber. Once it clicks into place, reinsert the eraser after detaching the old cassette.