A man walks into a hardware store and picks up an item. He asks the clerk “How much for 1?” The clerk says “Fifty cents”. “How about for 36?” asks the man. “That would be $1.” says the clerk. “Well then, how much for 105?” asks the man. “That would cost $1.50.” says the clerk. What was the man trying to buy?

Too easy? Try this one:
Take 10 objects and place them so that they form 5 rows of 4 objects each. The rows will be all the same length and the spacing between adjacent objects is all the same.