How to make gravel fall behind you in minecraft

In your house make a 3 by 2 entry way.

One block behind the entry way place a block that lines up with the edge of the entry way. Place tripwire hooks on those two blocks and use string to connect them.

Build a spiral staircase and stop 3 blocks above the ground. Place redstone on those blocks like this.

Make a four by seven square missing the side that is closest to the door. Place redstone on the blocks so that the two tripwires are connected.

Place sticky pistons on the inside of the square with the sticky part facing the entry way.

Throw diamonds or something somewhat valuable that somebody would want all around the inside. Make sure that one of the items is on top of the middle part of the string.

Place stone on the sticky end of the pistons.

Place two layers of gravel or sand (I prefer gravel for this because it's more creep looking) on top of the stone. DON'T TOUCH THE TRIPWIRE AFTER THIS OR YOU'LL TRIGGER THE TRAP!!!!!!!

Build a roof for the building without touching the tripwire or the valuable items.

Make the entry way a two block high one block wide entrance so that the person walking into the building won't see the tripwire.

TA DA!!! Now go troll your friend and trap them!!!!