Incomplete question. However, I assumed you are referring to the comments made in the online animated video (Chemical and Physical changes) by celebrity food scientist E. Mulson.
Explanation:
Recall in the video animation that E. Mulson defines physical reaction as occurring when "molecules in a substance stay the same, both before and after".
An example he gave was that of an orange getting juiced, the molecules remain the same before juicing, and after.
