The facts of life
Definition 1 of 2
Pro Tip 1/3Sensitive Topic
Consider the listener's age or relationship before explaining reproductive facts. 
Aunt Sue thought I was old enough to hear about the facts of life.
Pro Tip 2/3Be Euphemistic
Use this phrase as a gentle substitute for directly mentioning sex or reproduction. 
It was awkward when the teacher began discussing the facts of life in class.
Pro Tip 3/3Cultural Variance
The approach to this topic can vary between cultures; some may be more open than others. 
In some cultures, talking about the facts of life is taboo.