I go down six steep stone steps from the street to reach the laundry. At the entrance to this basement establishment there is a sign: "Watch your head!!!" (Yes, three exclamation points, suggesting numerous encounters of customer heads with the door frame.)
Good advice for me because at 6-ft., 3-1/2 in., I hit my head often, especially on doorways, chandeliers, street signs, kitchen cupboards - and low-hanging salamis in gourmet food stores.
My godson Tom is 6 ft., 10-1/2 in. He almost never hits his head. (Note these measurements to the half inch. We tall people are a vain lot.)
I theorize that middle-range tall people like me hit their heads more often than extra-tall people like Tom, since the latter group remains vigilant about obstacles.
I tested the thesis on Tom. He confirmed it and responded by e-mail: "The more often one has to duck, the less likely one will fail to do so."
I extended my research to historical figures. Lincoln stood 6 ft., 4 in. I don't recall reading that he ever bumped his head.
Perhaps he did, but his biographers did not consider it worthy of mention.
Peter the Great stood 6 ft., 7 in. Surely, I reasoned, he bumped his head.
Not at all. On the marshland where he founded St. Petersburg, there was nothing to bump his head against. The city did not exist yet. The greater danger for Peter lay at his feet: sinking into the marsh.
Called the "czar-carpenter" by contemporaries, this builder of St. Petersburg made certain his palaces had high doorways through which he could pass unscathed. Woe to the courtier with low doorways who invited Peter to his palace. He would learn that Peter was quick to anger. (On the subject of Peter's temper, read Nikolai Gogol's short story, "The Overcoat," where on a St. Petersburg street a phantom - Peter himself - frightens a policeman by shaking his fist at him and demanding, "What do you want?" The policeman flees the scene.)
Personal experience with doorways and other objects, as well as my own research, lead to an inescapable conclusion: "Watch your head!!!"