Specific Gravity
Specific gravity is a number that tells you how heavy a liquid is compared to plain water.
Think of it this way
Pour cooking oil into a glass of water and the oil floats on top. The oil sits above the water because oil is lighter for the same amount. Specific gravity measures this — it compares how heavy a liquid is to water. A liquid with specific gravity below 1 floats; above 1, it sinks.
Explaining specific gravity by grade level
Some water is heavier than other water. When you add salt to warm water, the water gets heavier. A heavy ball sinks in plain water but floats in salty water. The salt makes the water thick and strong enough to hold things up.