There are many periodic table jokes. Some jokes appeal to chemists and those familiar with chemistry. Others appeal to a wider demographic because they do not require and specialized knowledge. One example is: "If H2O is water, and H2O2 is hydrogen peroxide, what is H2O4?" The answer is, "Drinking!" This joke is an example of one that doesn't require any specific knowledge of chemistry.
A joke requiring some scientific knowledge is this: "Helium and some friends walk into a bar. The bartender looks Helium up and down carefully and asks, 'Are you a noble gas? Because we don't serve noble gases around here.' Helium doesn't react." To find the humor in this joke, the reader must know that noble gases are inert and do not react with other elements.
The periodic table consists of 118 chemical symbols, and many of the most common periodic table jokes are plays on combinations of these symbols. These jokes are similar to puns and require an audience familiar with the chemical symbols. For instance: "Oxygen and potassium were nearly divorced, but lately they have reconciled. They're OK now." This joke requires the reader or listener to know the chemical symbols for oxygen (O) and potassium (K).