Sweet and Low

At age 10, Nob Yoshigahara won the grand prize in a newspaper competition with this puzzle:

Given a sack of sugar, an unbalanced scale, and two 5-pound weights, measure exactly 10 pounds of sugar.

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Put both weights in one pan and pour sugar into the other until it balances. Then remove the weights and pour sugar into the first pan until it balances again. The first pan will contain exactly 10 pounds of sugar.