You shouldn't move someone's seat.
Donny is a practical joker who's looking to prank his coworker Frank who momentarily left his seat in the lunch room to go get food.
Donny wants to make people laugh.
Donny replaces the salt in the shaker with sugar, which Frank pours on his fries when he returns to the table to sit at his unmoved seat.
Donny moves Franks chair out from under him when Frank returns to sit down and Frank falls over, sending his food flying.
Coworkers are entertained by Donny's antics when Frank realizes what happened and spits his food out.
Coworker's chuckles turn to gasps as people realize Frank was seriously injured by the prank.