FOUR RIDDLES

1. What sign was posted on the door of Pavlov's famous institute?
2. What do you call the bill from an acupuncturist?
3. What can be first but not last, old but not young?
4. What is the most common blood type of poor spellers?

Answers:

1. Please knock. Do not ring bell. 
2. Sticker price
3. A hand 
4. Type O

Credit:
#2: Sarah Keller 
#4: Paul Guttormsson