Answer:
A precious stone, as clear as diamond. Seek it out while the sun's near the horizon. Though you can walk on water with its power, try to keep it, and it'll vanish in an hour.
What do you have when you're sitting down that you don't have when you're standing up?
What does everyone know how to open but not how to close?
What has 4 legs in the morning, 2 legs in the evening and 3 legs in the night?
A precious fluid, thicker than water.
The warmer I am the fresher I am.