Two men went to a pet store to buy a puppy. The first man paid $10 and the clerk asked him if he would like a poodle, a labrador, or a golden retriever. The man chose a poodle. The other man gave the clerk $10. The clerk gave the man a golden retriever. The clerk and man both said nothing. How did the clerk know what the man wanted?
The prices for the dogs were: Poodle $8, Labrador $9, Golden Retriever $10. The first man put down a $10 bill so he could chose what he wanted. The second man put down $10, one $5 bill and five $1 bills therefore the clerk knew what the man wanted.
You may enter, but you may not come in,
I have space, but no room,
I have keys, but open no lock.
What am I?