What kind of denominators make a decimal repeat?

A decimal repeats when the denominator of a fraction, in its simplest form, contains prime factors other than 2 and 5. This occurs because only fractions with denominators that are products of these prime factors can be expressed as terminating decimals. For example, the fraction 1/3 has a denominator of 3, which leads to the repeating decimal 0.333..., while 1/4 has a denominator of 4, resulting in the terminating decimal 0.25.