Archimedes of Syracuse (Sicily) was a great Greek mathematician, physicist, astronomer, & inventor (287–212 BC). 

He had designed a gigantic ship like the Titanic in the third century BC by using the principle of bouncy and giving the idea of density. 

Different Objects have different Densities

If you take the same volume of different substances, it is not necessary that they have the same amount of mass.

Density of Helium 

A helium-filled balloon moves upwards because the density of the helium gas is less than the density of air.

Density of Wood 

The density of the wood is less than the density of water, so it keeps on floating on the surface of the water/liquid.

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Density of Metals

The density of a metal object is more than the density of water, so when we keep it at the top surface of the water, it sinks into the water.