They're a vital part of our diet and we couldn't survive without them, but too much can kill us – fats.
Despite their bad press, fats are an important part of our diet.
Fat should make up about 30% of our diet
They perform four main jobs: to store energy, provide insulation, build cell membranes, and dissolve and transport certain vitamins.
But some types of fat are better for us than others.
And the crucial differences are the result of their chemical structure.
So what exactly are fats made of?
They all contain just three elements - carbon, hydrogen and oxygen - arranged into a distinctive looking molecule with three long "legs."
These three "legs" are fatty acids, long chains of carbon and hydrogen atoms, with oxygen atoms at just one end.
They're joined via their oxygen atoms to a short chain of just three carbon atoms.
It's the chemical bonds in the long chains that affect the properties of the fat.
Saturated fats, normally from animal sources like meat and dairy, have no double bonds between the carbon atoms in their chains.
This means they have no more capacity to bond with other atoms - they're "full-up" or saturated.
Saturated fat molecules:
Only have C-C single covalent bonds
The chains in adjoining fat molecules can pack tightly together, so saturated fats are normally solids at room temperature.
In contrast, unsaturated fats have at least one double bond between the carbon atoms in their chains.
Unsaturated fat molecules:
Have some C=C double covalent bonds
Fats with just one double bond are monounsaturated, and with more double bonds are polyunsaturated.
Vegetable sources like sunflower and olive oil, and oily fish, are rich in unsaturated fats.
The double bonds mean that "kinks" in the chains form, so the molecules can't pack closely together and unsaturated fats tend to be liquids at room temperature.
It's saturated fats, mostly found in animal sources, that can raise cholesterol levels and clog our arteries, leading to strokes and heart disease.
So getting your fats mostly from vegetable sources and fish can help keep your body healthy.