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                <h1>I'm a header.</h1>
<h3>I want a really giant header and a really tiny paragraph. So I set my body font-size to be 12px and use ems to change the font-size in the child elements, which are h1 and p. Ems are a relative unit, so they scale according to what you set the parent as. In this example, 1 em is the same as 12px, because I set the body font-size to 12px. 2em would be 24px. Play around with changing the font-size of the body, h1 and p to understand how this works.
<p>I'm a paragraph.</p>


  font-size: 12px;

  font-size: 4em;

  font-size: .5em;