Molecules are really important in chemical reactions. They are like the building blocks that change to form new things. When molecules bump into each other, they can mix up their atoms to create different substances. Let’s break this down into simpler parts:

1. Formation of Bonds: In a reaction, the connections between atoms in the starting molecules break apart. This takes energy. For example, when methane (that's a gas we can use for fuel, written as $\text{CH}_4$) reacts with oxygen (written as $\text{O}_2$), it makes carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$).

2. Rearrangement: After the bonds break, new connections are made between the atoms. This creates different molecules. In our example, the carbon and hydrogen from methane mix with oxygen to make carbon dioxide and water.

3. Energy Changes: Chemical reactions can either give off energy (we call this exothermic) or take in energy (that's called endothermic). When molecules change during these reactions, it often involves energy changes, which help us understand how reactions work.

To sum it up, molecules not only move around in a chemical dance but also control how one substance turns into another during reactions!