If you think you can order a gallon-size popcorn and stop eating when you’re full, think again: Visual cues, like portion sizes, have a huge effect on how much food we eat. 

In this classic study, groups of four university students ate bowls of soup for lunch. For two people in each group, the bowls of soup refilled automatically (and imperceptibly slowly) through tubes hidden underneath the table. For the other two in each group, a waiter came by if their portions ran low.

The students with the auto-refill bowls ate 73 percent more soup than the others, but didn’t think they had—and didn’t feel more full. All of the students also underestimated how many calories they’d consumed, and those with the refilling bowls were much further off the mark.

Thankfully, it’s unlikely that your local diner has auto-refill dinnerware, though family-style food spreads, passed hors d’oeuvres or (sneaky) free refills could snare you in a similar trap.