Piston knives are hazards in Silly Sausage in Meat Land.
The box is red. The piston knife has a long sausage-shaped hole in the blade when it comes out.
Piston knives are encountered in red boxes. When the Stretchy dog approaches a piston knife, it will extend out. After a piston knife extends out it will slowly retract back in spurts until it retracts completely. Contact with a piston knife will instantly kill the Stretchy dog.