Could someone spot mistake?

Hi, I am somewhat struggling with functions. I wrote similar one without the test strings, and seem to be working on my IDE. It yet doesn't return anything when I submit the task. Obviously, it is due to I changed "return" to "print". If I substitute print for return on line 6, it does not work. How could that be improved? (I still do not fully undertsand the concept of return)

1. def end_zeros(num):
2. counter = 0
3. while num % 10 == 0:
4. counter += 1
5. num = num // 10
6. print(counter)
7. end_zeros(3800)