This one comes from Sigmund Freud's book that analyses humour and its humour value is strictly dependent on your sense of humor. Two men with self-made fortunes decide to break into Society and do so by having their portraits painted by Society's most noted artist. At the gala celebrating the unveiling of the portraits, the leading art critic is asked to give his judgement. Looking at the space between the paintings, the critic asks where the Saviour is.
In the more traditional style: why can't a T-Rex clap it's hands? Because it's extinct.