I've been doing circuit board layout all week long, so last night at dinner when my friend gave me this puzzle to solve, I was surprised. It used the exact same part of my brain as circuit board routing, and it took me only a few seconds to solve.
Rules: You must connect 1 to 1, 2 to 2, and 3 to 3 by a line (the line can curve, it doesn't have to be straight). But the lines may not cross each other, and the lines cannot go outside the border.
It was so easy, I made a slightly harder version by adding 4 and 5 to the puzzle:
(sorry for the poor sketches, I'm learning to use my computer sketch pad. So far, the results look like something a 3-year-old kid would draw).