A fraction describes a part of a whole. The denominator (bottom number) tells us how many parts the whole is divided into (e.g. halves, thirds or quarters), while the numerator (top number) tells us how many parts we need to add or subtract.

Let's start by looking at how we add parts of a whole:

$1$1 part + $1$1 part = $2$2 parts

Now let's add in the language of fractions. You may have already looked at how to do this using a diagram or a number line. Let's say we wanted to add thirds together.

$1$1 third + $1$1 third = $2$2 thirds

We can write this using only numbers as well

$\frac{1}{3}+\frac{1}{3}=\frac{2}{3}$13+13=23

Now play around with this applet. What do you notice about the number on the numerator and the number on the denominator?