Everything is visible inside a glass box. When you PERFORM WHITE box testing , you can see the code and test it. But in BLACK box testing the tester does not know what is HAPPENING inside the code when he is GIVING a particular input. He only sees the output, but does not get to know how that output has come. In white box testing the tester can know what exactly is happening inside the code and why a particular out put is coming for a particular input. That's why it is called Glass box testing.

Everything is visible inside a glass box. When you perform white box testing , you can see the code and test it. But in black box testing the tester does not know what is happening inside the code when he is giving a particular input. He only sees the output, but does not get to know how that output has come. In white box testing the tester can know what exactly is happening inside the code and why a particular out put is coming for a particular input. That's why it is called Glass box testing.