Here's why:

* Methyl groups (CH3): The three hydrogen atoms on each of the three methyl groups are equivalent due to free rotation.

* Methine group (CH): The single hydrogen atom on the central carbon is unique and forms its own set.

You can visualize this by drawing the structure:

```

H

|

C

/ \

H CH3

\ |

C CH3

\ /

H

```

Let me know if you'd like to explore more about equivalent hydrogen atoms or other aspects of organic chemistry!