The picture shows 4 sets: a set of elephants, a set of books, a set of spoons and a set of apples. What do these sets have in common?

They all have the same quantity: five. The quantity of elephant is equal to the quantity of spoons, etc. To prove, we match elephants and spoons one to one. This way we compare two quantities finding which one is bigger. The other quantity is smaller, but if none is bigger, the quantities are equal.