The million dollar question

Apparently, my 6yrs old just found out about the word 'sex' and came asking. First I tried to wrapped my head around a suitable response while trying to find out where she heard it from, she quickly screamed 'school' cutting short my thinking time. Personally, I thought this was not a topic you say 'let's talk about this later, so I posed more questions to buy time; who mentioned it in school and how did they talk about it, what was the explanation you got? She tried mumbling some stuff, right then I had to say 'give me a moment we will talk about this later. What's the best way to approach this?