A toy based on a geared 5 bar linkage consists of a turntable on which you can put a paper disk.  Two gears are attached to the turntable and a pair of rods attached to the gears.  As you turn the turntable, the smaller gears turn which makes the rods travel.  A pen attached to the rods traces out a pattern

You can drag the big circle and watch the pattern be traced.  You can also drag one of the gears to alter their relative location.

You can change the length of the bars.