You get the notion to GRASP PEN WITH MOUTH. Good idea! But first you'll have to pick it up off the desk and into your maw.
You equip the last DISPOSABLE THUMB that you sort of forgot was in your pocket! You firmly attach it to your dewclaw so that you can enjoy TEMPORARY FINE MOTOR SKILLS. You'd better be efficient though, because you ARE supposed to throw these out after you are done with them.