Does "open up the tunnel" mean "open your mouth"? If so, is it a common expression?

Native speaker’s answer
Rebecca
Yes! Here it does mean "open your mouth". This is commonly used when feeding young children! If they don't like the food, adults try to make it fun by pretending the spoon is a plane or a train. In this case, the mouth is a tunnel! There are different variations, but they all have the same idea. So SpongeBob treats Squidward like a child who doesn't want to eat their food and says, "open up the tunnel". Ex: Here comes the train! Open up! Mmm, yummy. Ex: The only way I can get my child to eat vegetables is to pretend the spoon is a plane!