.. Heating the cauldron

You would need something that would fire it.

Something that would breath some fire into it. Read the book about "Dangerous Creatures".

There is a dragon upstairs. You can jus take it from its cage.

Did you read the book?

It says the will consume kerosene or paraffin to breathe fire.

Candles have paraffin but something else, also.

Look at the stuff in your inventory.

The crayon is made of paraffin. Feed it to the dragon. It can now breathe fire. Use the dragon on the cauldron.