why an egg can float on the water?

Originally, an egg is just sink in water, because weight of the egg is just a bit larger than weight of the water displaced by the egg shell. After the egg is stored for a prolong period, e.g a month or more (i.e. not so fresh), some composition of the egg white is consumed, then the weight becomes lesser and lesser. Eventually the egg will float on the water.

The larger end of the egg contains the air cell that forms when the contents of the egg cool down and contract after it is laid. Chicken eggs are graded according to the size of this air cell, measured during candling. A very fresh egg has a small air cell and receives a grade of AA. As the size of the air cell increases, and the quality of the egg decreases, the grade moves from AA to A to B. This provides a way of testing the age of an egg: as the air cell increases in size, the egg becomes less dense and the larger end of the egg will rise to increasingly shallower depths when the egg is placed in a bowl of water. A very old egg will actually float in the water and should not be eaten.
I hope this can help your understanding. :)

Not the weight, is the density!!!

because the egg is lighter than water