Oxidation
Oxidation is a chemical reaction where a substance combines with oxygen, like when lemon juice on paper turns brown from heat.
Think of it this way
Cut an apple and leave it on the counter. The white flesh slowly turns brown as oxygen in the air reacts with the apple's surface. This is oxidation. The same thing happens when iron rusts or butter goes rancid — oxygen bonds with the material and changes it chemically.
Explaining oxidation by grade level
You can write a secret message on paper using lemon juice. The writing is invisible at first. When a grown-up holds the paper near something warm, the message turns brown. That happens because the juice reacts with the air and changes color. The heat speeds up this change so you can see it.