In water, is an egg more like a boat or a rock? The answer depends on what's in the water.
Try this experiment to see for yourself. And do it near the kitchen sink, because you might spill some water.
Use a spoon to lower a coin, an egg, and a piece of cork into a glass of water. (If you don't have any cork, try a piece of light plastic packing material.)
The egg and coin sink. The cork floats.
Remove the objects, and add two tablespoonfuls of salt to the water. Stir until the salt dissolves. Now lower the objects into the salt water.
The coin still sinks, and the cork still floats. But the egg does not sink. Instead, it floats near the top of the water. (If it doesn't, dissolve more salt into the water.)
Using a tall water glass, combine the two experiments into one. Fill the glass one-half full with salt water.
Now pour a layer of tap water on top of the salt water without mixing them up. Here's how to do it: Unfold a paper napkin, and put one corner of it down into the tall glass so that the napkin covers the surface of the salt water. Hold the napkin in place, and slowly, steadily pour in tap water until the glass is almost full. Carefully pull out the paper napkin.
Now use a spoon to lower the objects into the water. The coin sinks, of course, and the cork floats. But now the egg sinks to about the middle of the glass. It goes to the bottom of the tap water but floats in the salt water. Push down on it gently with a spoon to see it return to the same level.
How It Works
Eggs are denser than tap water. That means an egg weighs more than an egg-sized amount of tap water. In the same way, a nail is heavier than a wooden stick of the same size because steel is denser than wood.
In tap water, the egg sinks like an air-filled balloon dropping through the air. But salt water is denser than the egg. Here, the egg rises like a helium-filled balloon in the heavy air around it.
Of course, you can't make water light enough to make a cork sink or dense enough to float a coin.