Hold a pencil and drop it...Viola!!! a Gravitron.
Step 1) Take your pencil along with you to an amusement park.
Step 2) Buy some tickets.
Step 3) Find the spinning puke machine where the floor drops out from underneath your feet while you are left plastered to the wall.
Step 4) Hold out your pencil and try to drop it. It sticks to the wall right alongside you (probably after it hits you in the face).
Viola!!! No Gravitons needed to explain what happened.