This isn't actually a puzzle, but it is something you should generally look out for throughout the game. You will come across the waterfall pictured below on your way up a mountain less than an hour into the game, and though you might just pass through, you can actually walk into the wall here to find a secret cave and some treasure. You should look out for secret passages wherever possible, sometimes it'll basically look like a normal wall tile but walking into it will reveal something new.