"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 best known as:

This increase in desire for more, after eating a first one, is best known as priming.

Explanation:

Priming refers to a technique in which being exposed to one stimulus will influence the individual's response to future stimulus, without much intention or conscious thought.

In this case, the individual is exposed to a particular substance and this will influence how he/she responds to being exposed to this same substance in the future. Since this response doesn't have to be a conscious one, the individual is most likely to eat more than one of the substance.