After Nancy gets into the locked evidence locker --

-- and reads the Search Warrant Report in the case file about the fire, she'll discover that part of the evidence against her is the fact that she had gasoline on her jeans from mowing the lawn the day before. She then asks her friends to get a sample from the fire to test for accelerants and see what was used to make it spread so fast.

First, you need a small glass bottle or tube with a lid. No problem. There's one to be found --

-- in the antiques store, next to the numbers game on the right counter. It even has a "free" label on it, so grab it right up.

Next, you need to get into the Old Town Hall.

Once inside, look for the point of origin -- that is, the place where the first flames appeared. In real life, you'd be looking for the place the scorch marks fan out from --

-- and that's true here too. But you don't really need it, because Nancy was standing right there looking at it when it burst into flames.

Go to the melted thermostat and use the bottle on the wall above it. Give it to Nancy, and you're done.