With four cuts, Dissect an Equilateral Triangle into a Square. First proposed by Dudeney (1907) and discussed in Gardner (1961, p. 34) and Stewart (1987, p. 169). The solution can be hinged so that the three pieces collapse into either the Triangle or the Square.
See also Dissection
References
Gardner, M. The Second Scientific American Book of Mathematical Puzzles & Diversions: A New Selection.
New York: Simon and Schuster, 1961.
Stewart, I. The Problems of Mathematics, 2nd ed. Oxford, England: Oxford University Press, 1987.