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We hear about calories all the time: How many calories are in this cookie? How many are burned by doing 100 jumping jacks, or longdistance running, or fidgeting? But what is a calorie, really? And how many of them do we actually need? Emma Bryce explains how a few different factors should go into determining the recommended amount for each person.

Lesson by Emma Bryce, animation by Qa'ed Mai.