"I'll bet you can't eat just one" refers to the fact that if you eat one bite of a particular substance you get the overwhelming desire to eat more of it. This increase in desire for more, after eating the first one, is known as

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Priming is a psychological phenomenon in which exposure to a stimulus (in this case, the first bite of a particular substance) increases the desire for more of that substance. This can lead to overeating or addictive behaviors.

"I'll Bet You Can't Eat Just One" Refers to the Fact