How economists view the world:
1) Individuals are all walking in a line
2) Individual i drops a $20 bill
3) Individual i + 1 says, "oh sweet, a $20 bill!"
3) Individual i + 2 do nothing, since such a state of dis-equilibrium normally doesn't exist
A more realistic viewpoint:
1) Repeat step 1 and 2 from before
2) Individual i + 1 says, "hmm, that guy dropped a $20 bill. Does he know something I don't know?" Individual i + 1 drops a $20 as well.
3) Individual i + 2 says, "whoa, those two guys just dropped a $20 bill. That must be a good investment strategy!" Individual i + 2 drops a $20 bill as well.
4) Repeat n times.
5) Individual i + n says, "whoa, what a bunch of suckers!" Individual i + n starts grabbing up $20 bills.
6) Individuals i + n + 1 to i + n + k start grabbing up $20 bills.
7) Individuals i + n + k + 1 start thinking, "whoa, those guys made a ton of cash picking up $20 bills. I'm going to take out a bank loan and expect that I'll grab enough $20 bills from other suckers to pay it back and make fat stacks of cash!
And so it progresses...