What is the last way to kill your boss on whack your boss

Various versions of the game have either 16, 19, 20, or 24 (official site may have more eventually). There are 19 in the version "Don't Whack Your Boss."

1-16: water cooler, wall, wall hook, his pencil, your hands, chair, cup, briefcase,
scissors, keyboard, monitor, ruler, stapler, wastebasket, top cabinet, umbrella
17-19: wall clock, file drawer, letter opener
20 : golf club
21-24 : lamp, flower vase, phone, computer tower

The earliest version seems to have only 7 ways: hands, cup, monitor, ruler, stapler, wastebasket, and top cabinet.