You have to boil a single egg for exactly nine minutes.

All you have are two hourglasses, able to measure (in sand) seven and four minutes, respectively.

How quickly, using only these two hourglasses, can you boil an egg?

Let us denote the 4 minute hourglass as F and the 7 minute hourglass as S.

It is possible to boil the eg using the hourglasses F and S,- in

conformity with the tenets inclusive of the puzzle in terms of the

following steps:

(A) Commence with F and S running simultaneously and initiate the cooking proceedure. Run both F and S four 4 minutes.

( Time elapsed: 0 Minutes)

(B) F has run out and is thus reset. Remaining time in S is 3

minutes.(Time elapsed: 4 Minutes)

(C) After 3 minutes, F has 1 minute remaining. S has run out

and thus is now reset.(Time elapsed: 7 Minutes)

(D) After 1 minute, F runs out and is reset. S has 6 minutes left and is now reversed, so the hourglass S has now precisely 7-6 = 1 minute left.(Time elapsed: 8 Minutes)

(E) After 1 minute, S runs out. The egg is now fully cooked. (Time elapsed: 9 Minutes)