It's rude to wipe your boogers on things.
Tim is feeling congested during math class but doesn't have any tissues handy.
Tim wants to clear out his nasal passages.
Tim excuses himself to the bathroom where he uses a tissue to blow his nose.
Tim picks his nose and wipes his snot and boogers on the desk.
Tim feels relieved and can breathe more easily.
Tim feels embarrassed when other students see him and make fun of him for picking his nose.