AAON ( AAON) looks "toppy" right now. Shares of the heating and air conditioning stock have been the exception to the other names we've looked at today -- this stock has enjoyed supercharged performance in 2013, rallying more than 74% from the start of January. But that sprint could be coming to an end. That's because AAON is currently forming a double top, a price setup that's formed by two swing highs at the same level -- a move through $21.50 support signals that AAON is toxic and it's time to sell. That doesn't mean that this stock's outcome isn't a foregone conclusion from here; a move above those $26 previous highs negates the downside setup. >>4 Big Stocks on Traders' Radars Whenever you're looking at any technical price pattern, it's critical to think in terms of those buyers and sellers. Double tops, triangles, and other pattern names are a good quick way to explain what's going on in a stock, but they're not the reason it's tradable. Instead, it all comes down to supply and demand for shares. That support level at $21.50 is a price where there had been an excess of demand of shares; in other words, it's a place where buyers were more eager to step in and buy shares at a lower price than sellers were to sell. That's what makes a breakdown below $21.50 so significant -- the move would indicate that sellers are finally strong enough to absorb all of the excess demand above that price level. Wait for that indication before you sell.