Quicksand is basically just ordinary sand that has been so saturated with water that the friction between sand particles is reduced. The resulting sand is a mushy mixture of sand and water that can no longer support any weight.

If you step into quicksand, it won’t suck you down. However, your movements will cause you to dig yourself deeper into it.

With quicksand, the more you struggle in it the faster you will sink. If you just relax, your body will float in it because your body is less dense than the quicksand.

http://www.howstuffworks.com/quicksand.htm