What do you do with a coat hanger, some rubber bands, an Altoids tin, and ten minutes of free time? Why, you make a miniature catapult, of course. You can easily sneak this into just about any office ...

In his spare time—which was presumably limited—Leonardo da Vinci was fond of drawing plans for contraptions and weaponry, most of which were never actually built. But with a few common items, it’s ...