On day I decide to take a party of three for a ride in a Cessna 172: Two girls, about 15 and 7, and their mother, with of course the two girls in the back.

We're flying along when the plane starts to yaw. I dampen it with rudder, but I have no idea why it would happen.

It starts again. Again I dampen it with rudder.

A few minutes later it starts again. The nose is swinging back and forth through about 30 degrees and I'm close to freaking out, because I've never heard of any such problem - instruments are fine (except the slip indicator) - what the hell is going on? If this pendulum swing gets much worse it's going to destabilize the airplane.

On a hunch I take a look into the back seat, and the two girls are holding hands and swaying back and forth in unison to rock the airplane.

One thing to add to your pre-flight instructions about seat belts - this isn't a carnival ride, and don't try to turn it into one.