Over 2000 years ago the Greek philosopher Plato suggested that everything was made of four elements; air, fire, earth, water.
Another Greek philosopher, Aristotle, believed that an element was something that couldn't be broken down into anything simpler.
Scientists have now found out much more about matter, and we now have different ideas about elements.
Which one of Plato's four elements is not a substance?
A- Air
B- Fire
C- Earth
D- Water
