Fire is hot and dangerous.
So what kind of things can we express with the word fire?
If you play with fire, you're doing something risky or dangerous.
Leaving your car unlocked in the city.
If you play with fire, you're going to get burnt.
If you describe someone as on fire, it means they're performing amazingly.
Did you see Molly's presentation?
She's on fire today.
She's doing so well.
But if you're under fire, that means you're being criticised or attacked.
This comes from fire referring to bullets.
Bill was under fire because he kept turning up late to work.
What happens if you add fuel to the fire?
You make it bigger.
You make a bad situation even worse.
Don't argue with her now.
You'll only add fuel to the fire.
Practice these phrases and you'll be on fire in your English conversations.