I'm just thinking back to when I was a child (not that long ago...really). I have a brother who's six years younger than me and I remember this one time with him:
I was taking him to the beach, and he was jabbering away (he was about three or four at the time). This, of course, really annoyed me so I pointed at a gate that was at the end of the road and said to him
"See that gate there?"
"Yes" said he
"It's there to keep horses out that will come out and kill you by running over you. If you keep talking, they'll break though and get you. So, you better be quiet"
"Okay." he whispers back at me.
I'm pretty sure there was a bit more detail to my description. I recall something about The Horses galloping up and down the beach with bloodied hooves and an evil glint in their eyes. I was reading James Herbert at the time...yeah...
Looking back I was a seriously mean big sister. I mean, what kind of scheming, sadistic, cow does that to her brother whilst holding back a snicker JUST TO KEEP HIM QUIET!? Fortunately, he has no recollection of the incident, but it just goes to show how conniving a nine to ten year-old can be.
Moral of the story: Keep an eye on your preteens, you never know what nasty trick they're brewing up for their younger siblings.
Also, don't let your 9-10 year old find your stash of horror...