One of the main topics of elementary physics is the idea that every material is composed of "little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one other." These particles could be atoms or molecules. Atoms are the smallest part into which any material can be divided, and when several atoms are joined together, molecules are formed. Some interesting experiments that do not require sophisticated equipment have been performed to estimate the size of such atoms or molecules. One of these early experiments was conducted by Lord Rayleigh (1842-1919), which consisted of a small drop of oil spread to form a circular patch on the surface of the water. With a few simple calculations it is possible to determine the size of the oil molecule composition and therefore to provide an estimate of the diameter of the carbon atom. The main aim of this article is that students, from middle school to high school, even at the undergraduate level, gain a quantitative understanding of the size of atoms by performing a simple experiment easily designed within the classroom.