Good Riddles

Question: sedentary stop + sudden start = ?

Question: Looking at my face, I'm no more than six.
Look at the rest of me and I'm 21.
You'll see my twin and me in Las Vegas.
Say my name and you'd be uttering a threat.

What am I?

Question: Mom and Dad had four girls. Each girl had one brother. How many people are in the family?

Question: Twelve pears hanging high, twelve men passing by. Each took a pear and left eleven hanging there. How can this be?

Question: What can you smell but not see. And can be deadly, too.