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Which Twin Is Older?

A simplified account of special relativity and the twins paradox.

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Take a Message Soldier

A messenger runs from the rear to the head of a marching column and back. When he gets back, the rear is where the head was when he set off. What is the ratio of his speed to that of the column?

Stonehenge

Age 16 to 18 Challenge Level:
Consider the movement of the stone relative to the logs and of the logs relative to the ground.