This is Why Sheldon always Knock at the Door 3 times
Well in The Big Bang Theory Season 10 Episode 5, Sheldon reveal the reason behind 3 Knock rule.
The Episode titled "The Hot Tub Contamination" Sheldon sat with Penny while having an Ice Cream in a Parlor, he opens up his traumatizing incident with His Father.
Things do not run smoothly when Amy moves in with Sheldon, and Leonard and Penny have to intervene; Howard and Bernadette find unexpected guests at their house when they decide to cancel their vacation plans.
Source: The Gup
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