In 1736, in the city of Königsberg (in what is now Russia), there was a puzzle that people talked about constantly. The city was built on a river, with two islands and seven bridges connecting everything together.
The question everyone was asking: can you walk through the city and cross every bridge exactly once? Not twice, not zero times — exactly once each. People tried and tried, but nobody could do it.
Then a mathematician named Leonhard Euler came along — and instead of just trying random routes, he asked a deeper question: why is it impossible? His answer changed mathematics forever.
Try it yourself first! Click the bridges (the numbered lines) in order to walk across them.