Why didn't I think of that?

I recommend Patently Silly, a blog by comedian Daniel Wright which collates ridiculous patent applications. The solo-operable see-saw is a typical example:

"a seesaw plank can be a dangerous object in the hands of a mischievous child who may abruptly pull down on one end when another child is passing by the opposite end, causing the opposite end to rise quickly and potentially striking the passing child."