The first sign of senility 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.

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

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. | No sources found for Erdos stating this, but if he did it is an expression made by many others before him.