Okay this one is from memory a long time ago.
A father and his son are in a car accident and taken to the hospital in serious condition. They both go to theatre. 5 Minutes later the surgeon comes out of the theatre a says "I cannot operate on this boy, he is my son".
Who is the surgeon?
Although a definitive answer would of course require further measurements, published species-wide averages of wing length and body mass, initial Strouhal estimates based on those averages and cross-species comparisons, the Lund wind tunnel study of birds flying at a range of speeds, and revised Strouhal numbers based on that study all lead me to estimate that the average cruising airspeed velocity of an unladen European Swallow is roughly 11 meters per second, or 24 miles an hour.
style.org/unladenswallow/