Archer Aviation Inc. (ACHR) had a big day Monday, surging almost 20% after announcing a partnership with defense contractor Anduril. But on Tuesday, the stock took a breather, consolidating near a price that could cap any further gains.

The stock is now hovering around $5.50, a level that used to be a floor. Now it might be a ceiling.

Here's the deal: $5.50 was a support level for Archer. Support is a price where there's a lot of demand — think of it as a trampoline. When the stock falls to support, buyers pile in and the selloff stops. That worked for a while. But eventually, sellers overwhelmed buyers, and the stock broke below $5.50. When support breaks, the people who bought at that level are suddenly holding losing positions. They tell themselves, "If it ever gets back to $5.50, I'm selling to break even."

And that's exactly what happens. When the stock rallies back to the old support level, all those unhappy former buyers place sell orders. That flood of supply turns the old support into resistance — a price ceiling. You can see it on the chart: $5.50 was support, now it's resistance.

So Archer's big rally is running into a wall. The stock needs to break through $5.50 with conviction to prove the resistance is gone. If it can't, the rally might fizzle. Traders who can spot these levels have an edge — they know when to take profits or wait for a breakout.

For now, Archer is at a crossroads. The Anduril deal is real news, but the chart says the easy money may already be made.