Volume is how much space is taken up. If you are finding volume of a cube you do L x W x H (length, width, and height)
Mass means how heavy it is. You measure mass on a triple beam balance.
Density is if something sinks or floats to find the density of a cube you need to do mass divided by volume.
For example, if you have a cube with a mass of 8 and a volume of 15.625 you would divide it and get 0.512.
If you are finding density of a cylinder you measure the mass on a triple beam balance. To find volume you need to put water in a graduated cylinder then put the cylinder in and the amount it has increased is the volume. Finally you need to do the mass divided by the volume.
