Blackboards in Porn: Critiquing the background set décor in those skeevy teacher/student porn scenes. This is the reason the Internet was invented. (NB: Surprisingly worksafe, though the illustrations still feature skeevy teacher/student actors.)
Education cutbacks are clearly hitting hard – this establishment can only afford a periodic table with four elements, and a hand-drawn one at that. Though not displayed in the correct order, all the information about the elements is correct, with the exception of lanthanum’s atomic weight. According to IUPAC, to three decimal places this should be 138.905 instead of 138.906 (actually 138.90547, so the rounding error is not great). Analysing the history of IUPAC’s regular redefinitions of elements’ atomic weight (as scientists become more accurate in their measurements) it can be deduced that not only is this teacher using a hand-drawn periodic table containing only four elements, but that it is at least seven years old (lanthanum's atomic weight was first published as its current value in 2005) and possibly dates back as far as 1969 (when 138.9055 replaced the previous figure of 138.91). Highlighting tiny discrepancies in atomic weight may seem like nitpicking, but it is this kind of fourth decimal place analysis that led to the discovery of deuterium.
via Blackboards in Porn.