Cannot subtract to negative answer

I was hoping this would be useful in teaching math concepts but I found it lacking and only teaching how to crunch the numbers. I wish the subtraction was more visual (like with a number line) Also, it does not allow you to subtract a larger number from a smaller number to yield a negative answer. It just reverses the order no matter how your enter it so it is a positive question.

Skateasian about Math Animations (Grades 1-8)