Kia found out her Mum had cancer from smoking when she was just seven years old.
Her Mum had been smoking for 25 years.
Now Kia's 11 and her Mum is not going to get better so she and her sister are appearing in an anti smoking advert to try to get others to kick the habit.
Kia told Newsround: "It's really hard. I'm 11. She told me when I was 7. I burst into tears.
"I thought my mum wasn't going to be there for me. It's hard not to cry. I tried to think happy thoughts.
"I want people to stop smoking. It's stupid to start."
Every year 114,000 people die because of smoking.
One in 10 girls and one in 14 boys regularly smoke.
If you smoke when you're young you're three times more likely to get hooked for life.
Kia and her sister are hoping that telling their story will make parents understand the damage smoking can do to their families.