How to make the perfect "third wheel"

I admit I have not tried this, but it just struck me:

You know the kind of robot.. well - like the "start here-project"; You have 2 wheels, and perhaps you need something to drag behind?

Well, the perfect solution would not be half a tennis table ball, or that kind of thing. The perfect would be a little wheel dragged on an open L-shape, a caster.. yes.. but if you do not have a such, this is it: Use a pearl!!

The bigger you can find it, the better, of course. Huge plastic pearl, stick a pin through the hole (perhaps drill it out first), and you have a near perfect caster :) Mount it so that when driving forward, the perl is spinning, and when going to the side, it will slide..

Why on earth did I never think of that before? :D

It does not need to be a pearl, you often get little hard plastic balls that has a hole in them. Little black thingeys, they are mounted on a strings or something to do some stringey-stuff or whatever :) That would be perfect!

I am a collector of hard round objects with holes in them from now on!

PS: No twss in here, I have written it so it cannot work!