in one of the most interesting book "Number: The Language Of Science" by Mr. Tobias Dantzig (yes _that_ Dantzig, father of Mr. George Dantzig, the one who discovered Simplex Algorithm) talks about the "number sense", and has the following (heavily paraphrased) anecdote about a squire who want to shoot a crow that has nested in the watchtower of his estate:

    Whenever he goes into the tower, the crow flies off and won’t come back until he has 
    left. So he tries a trick: two men go in, one comes out, and the other stays hidden. 
    The crow isn’t fooled and waits until the second man leaves too. The same happens with 
    three men and with four. Only with five men, four leaving and one staying, does the 
    crow lose track and return to the nest.