Found Something!

Hey, what's this? You just happen to notice a piece of paper, covered with mud, near the bottom of the wall of the beaver house. Carefully brushing some of the mud off, you see that the ink has run, but the writing is still legible:

Dear Wilhelm,
Please take care of this riddle for me. I fear that my cave may be a tad damp for the keeping of paper. It is:
"Seen from inside, I am everything. Seen from outside, I am not even real."
Sincerely,
Alfred

You decide to carry this around, so you put it in your pocket, an action you regret as soon as the slimy mud permeates through to your leg. You wisely and confidently change your mind, placing the paper carefully back where you found it.

Go back to looking at the house.
Knock on the house.
Push off, enjoy the lake a bit more.