You are telling your friend that organic molecules are all made up of carbon backbones with hydrogens. she doesn't understand how there can be so many different organic molecules if they all are made up of the same basic components. you explain that organic

 They possess different functional groups.

Explanation:

Organic compounds are molecules composed of many carbon atoms bound together and in turn bound to hydrogen atoms. These molecules form the lipids, proteins, carbohydrates and nucleic acids that constitute all living beings. The reason why there is so much variety of organic molecules is because each one can have one or several functional groups in different positions, which gives them unique properties.

So a functional group is a group of atoms linked in a certain way, which present a specific physical-chemical structure and properties that characterise the organic compounds that contain them.

Examples are:

its temperature is increasing. this causes molecular vibrations to increase.