That glimmering ice in a beer ad could be diaper stuffing. Look closely: is that vanilla ice cream or mashed potatoes?

What you see in food commercials is not necessarily what you get, as “Tabletop,” a great new play at the Bessie Schoenberg Theater makes crystal clear. (Or is it plastic?)

In the show — about the art of beautifying food and drink for ads — a director is scandalized when a production assistant makes a swirly fruit shake from shaving cream and food coloring.

“Basically, you can’t legally substitute something else for the product you’re selling.” says playwright Rob Ackerman. “But background foods are fair game.”

Ackerman, 41, who works as a prop-master for commercials, films and television , told The Post a few tricks of his trade.

By the way, the show runs through Sunday. (Reservations: (212) 924-0077.)