IT'S ALL ABOUT DENSITY
Wondering why lemon floats in water? Here is the answer.
Osmosis (The movement of water from less concentrated solution to a more concentrated solution) makes the water enter into the lemon and brings down its concentration. Lemons are denser than limes which are small and green. The weight of the lemon becomes less than that of water. The outer skin of lemon is waterproof and acts as a barrier to keep water out, protecting the pulp inside.