"Daily trying to get to the bottom of the mysteries of Lake Hylia!"
At the Lakeside Laboratory, while Link is in the midst of acquiring the Biggoron's Sword, he finds that he needs the Eyeball Frog to create the World's Finest Eye Drops for Biggoron. After receiving it from King Zora XVI at Zora's Domain, he must bring it to the Lake Scientist on horseback for increased speed, as the Eyeball Frog spoils quickly. Teleporting will send the clock to one second.
Pieces of Heart
At the very top of the Lakeside Laboratory is a Piece of Heart. To get up there, Link can either plant a Magic Bean as a child and ride it as an adult or use the Scarecrow's Song and Hookshot up. Watch out for the Guays.
Once Link wins the Golden Scale from the Fishing Pond, he can enter the Lakeside Laboratory and use it to dive down as far as he can. Link will touch the bottom and hear the Zelda chime. Swim back up and speak with the Lake Scientist to get a Piece of Heart.