Take a glass and fill it with water. Now take an egg and put it in the glass. You will see that the egg sinks in the water.
Take another glass and fill it with water. Now mix some salt in it and put the egg in the glass. You will see that the egg floats in the water.
Now, pour out some of the salt water and slowly pour some fresh water in it. This time the egg floats halfway.
How does this work?
In the first case, we see that the egg sinks into the bottom of the normal water glass. This happens because the weight of the water, which the egg pushes, is less than the weight of the egg itself. But in the second case, we see that the egg floats in the salt water. This is because the weight of salt water is more than fresh water.
In the third case, when the salt water is mixed with normal water, the weight of both fresh water and the salt water is equal to the weight of the egg, resulting in the egg to float halfway.