Useful links:
http://cwis.nyu.edu/pages/mathmol/modules/water/density_intro.html: Volume, Mass and Density Part I: Volume
http://cwis.nyu.edu/pages/mathmol/modules/water/dpart2.html: Volume, Mass and Density Part II: What is the mass of the block?
http://cwis.nyu.edu/pages/mathmol/modules/water/dpart3.html: Volume, Mass and Density Part III: Finding the density of an object
http://cwis.nyu.edu/pages/mathmol/modules/water/dpart4.html: Volume, Mass and Density Part IV: Challenge Questions