One of these four numbers does not share the property of the other three. Pick the odd one out: 216, 343, 400, 512.

216 is 6 multiplied by itself three times, 343 is 7 taken three times and 512 is 8 taken three times, so each is a perfect cube. 400 is $20\times 20$, a perfect square and no cube at all — the cubes on either side of it are 343 and 512. It is the trap precisely because it is the roundest and tidiest-looking number of the four, and roundness is not the property being tested.