Good Riddles

Question: what makes a snap sound and capture the happy moments of you ?

Question: sedentary stop + sudden start = ?

Question: What do you call a sad bird?

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?