The latest episode of Young Sheldon finally solved a mystery from The Big Bang Theory, seven and a half years later.

For those who don’t know, Young Sheldon is a prequel series to The Big Bang Theory and it follows the younger years of Jim Parsons‘ character Sheldon Cooper.

In the season 10 episode of The Big Bang Theory titled “The Hot Tub Contamination,” Sheldon explained why he always knocks three times before he enters a room.

He told Penny, “I was 13 years old, and on spring break from college. My mother was in bible study, I walked in the house expecting to find it empty, and I heard a sound coming from my parents’ bedroom. When I opened the door, I saw my father having relations with another woman… We locked eyes, I ran to my room, and we never, ever spoke of it.”

Well, Young Sheldon finally revealed who Sheldon’s dad was with in bed.

Keep reading to find out more…

It was actually Sheldon’s mom Mary!

In season 7′s fourth episode, which aired on Thursday (March 7), we learned that 13-year-old Sheldon came home from college and only knocked on his parents’ door one time. He opened the door to find them doing the deed.

Sheldon might have thought his father was with another woman though because his mom Mary was dressed in clothes she brought home from Germany, including a blonde wig. Sheldon’s parents were doing role play and when he walked into the room, his dad was saying, “Oh, Helga!”

Narrator Sheldon explained, “I never talked to my parents about what I saw that day. But from then on I added extra knocks so people could get their pants on,” according to TVLine.

Did you see that CBS has officially ordered a Young Sheldon spinoff series with two actors reprising their roles?