Banjo jokes are not in the stict sense of the word jokes. Most are an attempt to be helpful in seeking to guard the unwary from the dangerous effects that banjos have on the health and sanity of those who are unfortunate enough to be within earshot when they are being played. In issuing such warnings one may adopt an attitude of gravitas or one may attempt to reassure and educate the potential victim by injecting a modicum of mirth into the warning. I think I should rest my case (which does not contain a banjo).