The 20€ bill is the best bank note ever because you know it will cover whatever you've purchased even when you did not comprehend the total the cashier has just said to you. You simply hand it over with that meek smile that says, "I know how much that croissant and brioche costs and I perfectly understood what you said to me. It's just that this 20€ is simply the only thing I have."
Your smile most certainly does not say, "I am in a blind panic. My wallet is filled with ten pounds of strange change I cannot distinguish. In the hopes that this will cover the cost, I am handing you the largest bill in my wallet. If I had a 50€ note, I'd pass that to you instead, even though this baguette costs 70¢."