Sailboats providing their own wind

[kith]

At the time one could purchase from one of the scientific supply houses this so-called Newton's Third Law demonstrator that had a fan mounted on a cart, blowing towards the (rigid) sail that is also mounted on the cart. The lecturer could turn on the fan and demonstrate that it indeed didn't propel the cart.

I remember seeing such a device at a science fair a few years ago. The fair itself was outstanding and had many ingenious demonstration experiments for the vistors to play with (tacking with small land sailing vehicles, water containers which could produce bubbles on demand which then would sink a ship, etc.). Given this, I was really surprised to find a device which demonstrated wrong physics among them.