During Harry's first occlumacy class with Snape when Harry asked if it was like mind reading Snape snapped back saying that he was acting like a muggle. The pain that Snape felt when Harry asked the question was a pain that he felt often in the course of the past 5 years when ever Harry did something that reminded him of Lily.
It was back in their forth year they were in the library looking in books for homework help when they came across the word occlumacy after reading a few lines of the explanation Lily asked Snape the very same question Harry asked Snape that first lesson 'Is it like mind reading?' Laughing Snape answered 'No you cute muggle witch' with a small kiss on her cheek.' It is not connected to the muggle mind reading you are talking about. Here let me read the full explanation to you.' Remembering this and remembering that Lily was not alive because of him he snapped backed to Harry hating him not only for being James Potter's son but for being so much like the girl that he loved and lost.
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