It's quite important that you do not make the mistake of thinking that if you tell someone it is easy to do, because you find it easy to do, that does not make it easy for them to do.

With a kid, for example, you can tell them this and they will believe that it's easy for everyone, then try it and fail, and suddenly believe they are fucking stupid and it must be their fault entirely. Which makes them feel bad. Which might make them eat more, or become anorexic, or any one of a number of eating disorders.

So instead, realise that there are many factors here beyond energy in and activity out. Those are important, but not the whole story. Be kind, and take your time.