Assure them that you have plenty more. You will give them more as soon as they have the first five chewed and swallowed. Start the stop watch and allow the contestants to begin chewing. On a rare occasion a student will be able to chew and swallow more than five pretzels in a minute. Most will only be able to swallow two or three.
Most students completely overestimate their ability to eat the pretzels in a minute.
Predicting is a skill a person needs to develop to become a successful scientist. Students should notice how predictions change as more information is made available. The experience with eating a portion of a pretzel and information given about the additives should have motivated them to change their predictions. Scientists gather as much information as they can before making predictions. As new information is made available to them, they often change their predictions.