Can you draw a line through all of the edges in this picture?
Each side is broken into 2 or 3 edges, and there are also 7 edges inside that you have to cross. The line must be continuous, and cross each edge exactly once.
Hint: Try this with a piece of paper.
There is no possible way to complete the line, there will always be one edge left - or you have to cross an edge twice.
This puzzle is the same as the famous 'Seven Bridges of Konigsberg' problem first solved by Euler.
In that problem, the task was to find a closed path that crossed each of the seven bridges of Konigsberg (now Kaliningrad, Russia) exactly once.