Demonstrating how surface chemistry and self-assembled monolayers (SAMs) control the macroscopic properties of materials is challenging as it often necessitates the use of specialized instrumentation. In this hands-on experiment, students directly measure a macroscopic property, the floatation of glass coverslips on water as a function of modifying the terminal surface groups of the glass. The glass surface is chemically modified by the self-assembly of monomolecular layers formed by two organosilanes, 3-aminopropyltriethoxysilane and 1-octadecyltrimethoxysilane, which change the water contact angles. These SAMs alter the ability of the modified glass to support a mass, thus demonstrating that the bulk material property can be directly controlled by molecular surface chemistry. (Contains 3 figures.)