Every object has a centre of mass — the single point where all of its weight is effectively concentrated. For a symmetrical object like a ruler, that's right in the middle. For a lopsided object, it shifts toward the heavier side.

Here's the key idea: where the centre of mass sits relative to the balance point determines everything.

When you try to balance a plain paddle pop stick on your fingertip, the centre of mass is somewhere up along the stick — above your finger. Any tiny wobble shifts the centre of mass past the tipping point, gravity takes over, and it falls. That's called unstable equilibrium.

Now bend the pipe cleaner into an arch, attach it low on the stick, and clip the pegs to the ends. The pegs hang below your fingertip, pulling the centre of mass of the whole structure downward. If the attachment is low enough, the centre of mass drops below the balance point. 

Now when the structure wobbles, gravity actually pulls the centre of mass back toward the lowest point — and the stick corrects itself. That's stable equilibrium. The ping pong ball in [The Impossible Blow] finds the same kind of stable position — the airstream creates a pocket it keeps returning to, no matter how you nudge it.

The lower the attachment point, the lower the centre of mass, and the stronger that self-correcting force. Move the pipe cleaner up too high and the centre of mass rises above your finger again — and it falls.

Real tightrope walkers use exactly this principle. The long pole they carry droops at both ends, pushing the centre of mass of the whole system — walker plus pole — lower than it would be without the pole. 

Curiosity spark: What do you think would happen if you bent the pipe cleaner so the pegs pointed upward instead of hanging down — would it still balance?

Find out in The Crazy Scientist Lab.