Tell her that It’s just what happens when adults forget to turn off the lights, close the door and suddenly think ‘Damn! how did that happen?’ Stick to your Legos for now! Just kidding, by the way. Be very careful what you tell her because kids are unpredictable. It would be embarrassing if you're in a busy restaurant and she shouts, 'Mommy, what's sex?' So, answer her every question don't avoid it because kids are curious and smart.

Found these two websites for you : https://educateempowerkids.org/common-questions-kids-ask-about-sex-and-how-to-answer/