The 2 questions

>EXPERIMENT<
Place a fresh egg in a container of fresh water and the egg will sink to the bottom. Stir salt into the container of water. Replace the egg and this time it will float because you have increased the density of the water.

QUESTION:
Now, try to explain why your egg sunk in tap water but floated in salt water.

Salt water can make any object float easier because it is denser than fresh water, therefore it can support bigger objects.

QUESTION:
Why and how did the density of water change when salt was added to it?

A molecule of water has the mass of the hydrogen and oxygen it contains. A salt water molecule also has the additional mass of the sodium and chlorine atoms dissolved in the water. Density is a measure of mass per volume, so the higher the mass, the higher will be the density. Fresh water has a density of 1.0 while salt water has a density of 1.025. From this, you can see that salt water is slightly heavier than fresh water. The ratio between the two is 41:40.