This TI-81 decimal to fraction program will work... eventually. Have patience.
This program is also available through TI's own education web site. It is very inefficient. The main loop of this program keeps multiplying the input number by an ever-increasing counter. When the multiplication results in a number that's close enough to an integer, then the value of the counter is the denominator of the fraction we seek. Then the original input number is multiplied against this denominator to figure the numerator.
Try entering ".001" into this program. It will take a minute or more to get an answer.
I've found that the program on this web site is much more efficient: