During the 1920s, people had a taste for “Jackass”-style daredevil spectacles. Barnstorming pilots and wing walkers dazzled crowds at traveling aerial shows. Flagpole sitters, including Alvin “Shipwreck” Kelly, perched atop poles for up to 49 days in a row.
Harry Kahne, “The Man with the Multiple Mind,” was a daredevil of the brain. Kahne performed as part of vaudeville acts, doing complex calculations while carrying on conversations with the audience and writing out news headlines backwards. Occasionally, he would write different words simultaneously with each hand and foot and with his mouth.
The trick depicted in the photo below—a crossword puzzle solved while hanging upside down—was developed as the result of a dare. Compared to some of Kahne’s other feats, it seems almost simple. In this clip from 1936, for example, Kahne demonstrates how he can write the alphabet upside down and backwards, while also reciting his A-B-Cs out loud—in the opposite direction.
In a 1925 interview, Kahne said that he started teaching himself to do two things at once after being chastised once too often for daydreaming during school. He consistently argued that anyone who was willing to devote time to “mental gymnastics” would be able to do the same things he could. You can find PDFs of his “Multiple Mentality” course, should you wish to try.