title: What is meant by adding a column in bits?
body: <p>I am not able to understand what is meant by the following text, which I referred from a book:</p>
  
  <blockquote>
    <p>Consider the four two-bit numbers
    <code>00</code>, <code>01</code>, <code>10</code>, <code>11</code>. If you add up
    the one’s bit, you get an even number.
    Likewise, if you add up two’s bit, you
    get an even number. No matter how many
    bits in the number, if you add up a
    column, you get an even number.</p>
  </blockquote>
  
  <p>Specifically, what is meant by "add up one's bit" for <code>00</code>?  </p>