Oxidation and Antioxidants
Oxidation and antioxidants is how air breaks things down and how some vitamins slow that damage.
Think of it this way
Cut an apple in half and leave it on the counter. The open flesh turns brown as air attacks the surface. Dip a slice in lemon juice first, and it stays pale much longer. The vitamin C in the juice blocks the air before it can do damage.
Explaining oxidation and antioxidants by grade level
Cut an apple and watch it turn brown. That happens because air changes the fruit. Vitamin C can help stop the brown color. It works like a shield for the apple.