Question

Cotton is a solid still it flows on water

Answer 1

because it is lighter than water that is why it floats on water

Answer 2

density of cotton = 1550 kg/cm^3

density of water = 1000 kg/ cm^3

so density of cotton> density of water, but still cotton floats on water because generally cotton has air spaces in it....

to explain this, let me take an example

the density of bird is more than density of air but still birds are able to fly because they have hollow bones/ pneumatized bone filled with air sacs.

Hope it is clear!