“Morfran thrust his axe straight up. He pretty much seemed to have one sign for everything: poke a hole in the sky.”

Source: The Drunkard's Walk, Chapter 5, The Dueling Of Large And Small Numbers, p. 89

"Adventure's End" in The Norton Book of Sports (1992) edited by George Plimpton, p. 85
Context: It was too late to take risks now. I asked Tenzing to belay me strongly, and I started cutting a cautious line of steps up the ridge. Peering from side to side and thrusting with my ice axe, I tried to discover a possible cornice, but everything seemed solid and firm. I waved Tenzing up to me. A few more whacks of the ice–ax, a few very weary steps, and we were on the summit of Everest.
It was 11:30 AM. My first sensation was one of relief — relief that the long grind was over, that the summit had been reached before our oxygen supplies had dropped to a critical level; and relief that in the end the mountain had been kind to us in having a pleasantly rounded cone for its summit instead of a fearsome and unapproachable cornice. But mixed with the relief was a vague sense of astonishment that I should have been the lucky one to attain the ambition of so many brave and determined climbers. I seemed difficult to grasp that we'd got there. I was too tired and too conscious of the long way down to safety really to feel any great elation. But as the fact of our success thrust itself more clearly into my mind, I felt a quiet glow of satisfaction spread through my body — a satisfaction less vociferous but more powerful than I had ever felt on a mountain top before. I turned and looked at Tenzing. Even beneath his oxygen mask and the icicles hanging form his hair, I could see his infectious grin of sheer delight. I held out my hand, and in silence we shook in good Anglo-Saxon fashion. But this was not enough for Tenzing, and impulsively he threw his arm around my shoulders and we thumped each other on the back in mutual congratulations.

Source: Disgrace

Song lyrics, The Kick Inside (1978)

O'Keeffe's contribution (1939) to the exhibition catalogue of the show An American place (1944)
1930 - 1950

Source: Choke

Source: What's the Worst That Could Happen?: A Rational Response to the Climate Change Debate (2009), Chapter 1 "The Decision Grid" (p. 31)

Though Erdős used this remark, it is said to have originated with his friend Stanisław Ulam, as reported in The Man Who Loved Only Numbers : The Story of Paul Erdős and the Search for Mathematical Truth (1998) by Paul Hoffman
Variants:
The first sign of senility is when a man forgets his theorems. The second sign is when he forgets to zip up. The third sign is when he forgets to zip down.
As quoted in Wonders of Numbers : Adventures in Mathematics, Mind, and Meaning (2002) by Clifford A. Pickover, p. 64
There are three signs of senility. The first sign is that a man forgets his theorems. The second sign is that he forgets to zip up. The third sign is that he forgets to zip down.
Misattributed

Attributed in Paul Hoffman, The Man Who Loved Only Numbers: The Story of Paul Erdős and the Search for Mathematical Truth (1998)
This has also been attributed, with variants, to Paul Erdős, who repeated the remark.