There are two types of testing
1. Black box testing ( by giving inputs test the output according to the requirements)
2. White box testing ( testing source code )
Example:- Requirement for age drop down (1-100)
If you select the values between 1 -100 it should accept, else it should display error message.
But it is not possible to test all the values from 1 to 100(time consuming)
To ov ......