Grin and Bear it

My son is at times the most adorable, sweetest, funniest little human being I know, but then he can become the opposite. No two days are the same, I am at times baffled at how a 3 year old can be like an adult, its quite scary at times, that they learn things so quickly and are very quick to spot things, and point things out - things that maybe they shouldn't know, but its hard to draw the line on what they should be learning...I guess that's what they call growing up?