(Explanation for young children)
A molecule is what you get when two or more atoms hold hands and stick together. They can be the same or different types of atoms.
Go back to the LEGO idea. One LEGO brick is an atom. When you click a few bricks together, you’ve made a small shape like a car or a house or a bridge. That shape is a molecule. The shape can be made up of bricks of different colours, or it can be made up of just one colour brick.
Water is a good example. One water molecule is made of three atoms: two hydrogen atoms and one oxygen atom, joined together. Every drop of water you’ve ever drunk is made of billions and billions of these tiny three-atom shapes.
The same bricks can make different things. Oxygen atoms can pair up with each other to make the oxygen you breathe. But if you join an oxygen atom with a carbon atom, you get a completely different gas. Different combinations make different molecules, and different molecules behave in different ways.
Molecules are everywhere. The sugar in your cereal, the air in a balloon, the soap you wash with, and the smell of freshly baked bread all come from molecules. Some are tiny, with just two atoms, and some are enormous, with thousands of atoms. The molecules inside your body that carry your instructions for growing, called DNA, are some of the biggest.
So the simple version: atoms are the bricks, and molecules are what you build with them.