When pouring candles, why does the center of the candle sink surrounded by as the wax cools? whenever I pour a candle, after letting the wax cool I notice...

whenever I pour a candle, after letting the wax cool I notice the center of the wax (at the wick area) have sunken in. I don't know what to do to avoid this issue.
Answers:    I'm sure in attendance is a complicated explaination that involves molecular cohesion and such, but I solve the problem by saving some of the wax and doing a second pour to compress the dip.
It's called contracting. ya warmth up stuff it expands. maybe your wax is a bit too hot get hold of it just pourable.