Go around somewhat to the side- there's an area where the stones surrounding the spring are thinner. Stand there when tossing an egg. It'll pop back into your hands after a moment as a spa egg.
If you throw the egg in once it'll be a little yellowish and be a "spa-boiled egg".
If your throw it in a second time, it'll be white and in a grey/silver container and called a "boiled egg". The same looking as if you would boil it in your kitchen.