Don't tell mom story:
When I started to be sexually active, I experimented with girls and I didn't want my mom to know that I had been with girls... because she was against that kinda thing. She eventually found out, expressed her disappointment, and we moved on from the subject.
Don't tell dad story:
I started smoking at a young age, my mom knew, but we kept it from my dad because he would have freaked out about it. I was 16, but even at that age, my dad wouldn't let me get my ears pierced. My dad eventually found out as I was on break at work and he came to shop at the store I worked at and he seen me outside on break smoking. He told me, "When they start to slowly kill you and you have many health problems from them, you'll see that you made the wrong choice." and dropped it.
Don't tell mom or dad story:
When I started doing/trying drugs... of course I didn't want either of my parents to know. As all parents do, they eventually found out. It's not like I was addicted to anything, just trying things out... so it wasn't a huge deal, though I would get hounded every so often about smoking weed and stuff. And no... I don't do any drugs these days.