The rat holds on to that key. Try to get it.

Maybe you could exchange it to something.

In every adventure puzzle, when we are dealing with mice and rats, it is always that same thing.

Exact-o. Get it some cheese.

And, as usual, you need to make some cheese-like substance.

Cheese has a strong smell. What else has strong smell?

Unclogged drains, yes. Did you find a drain downstairs?

When you click on it you get some smelly mold.

Might be. Give it to the rat.

So, it should be... different color?

Right. There's some yellow paint at Anna's table, with the art stuff.

Paint the mold yellow. Give it to the rat to get the key.