Alone I’m Unknown
Alone I’m unknown,
With my twin I’m twenty,
And triplets make me a warning.
What am I?
The letter ‘X’. A single x is often used in math as an unknown, along with Mr. X. Two together (XX) make the Roman numeral 20, and three signifies a warning for adult only content, or a warning to other programmers of a problem in source code.
More You Take Away, The Bigger I Get
The more you take from me, the bigger I get. What am I?
A hole.
This is quite similar to It Cannot Be Seen and Weighs Nothing.
In Oven But Not Baked
What must be in the oven yet can not be baked?
Grows in the heat yet shuns the light of day?
What sinks in water but rises with air?
Looks like sand, but is fine as hair?
Yeast.
Celebrate, Hide Their Shame or Commit Crime
Some use me to celebrate,
Others use me to hide their shame,
Others use me to commit crime.
What am I?
Alcohol. People have a drink to celebrate, but also drink to hide their shame. Lastly, drunk driving is a crime.
A mask is also a valid answer. They can be used to celebrate Halloween or at dress-up parties, a mask can hide your shame and masks are used by thieves.
Update: This one has more valid answers than I thought. Money, drugs and night also work.