1/6 cup of water. In order to half something, you must divide by 2. When diving fractions, you must multiply by the reciprocal (opposite) so instead of 1/3 times 2/1, you would multiply 1/3 times 1/2. This gives you 1/6. To check this, divide 1/3 on a calculator. You’ll get 0. 33333333333. This divided by 2 is 0. 16666666666 which is 1/6.