Pay as you go:
So there are four roommates who share an apartment. As an agreement, each pays his share of money for rent and utilities into a single account. One of the roommates (or a rotating set, or whatever) then pays the bills from that account. Each roommate contributes his share from his paycheck on a set schedule based on his salary and when he gets paid, so Tommy gets paid weekly and contributes weekly, but Eddie is paid every two weeks and so contributes every two weeks. Whatever.
As long as everyone pays on schedule, the money is in the joint account and gets paid for the bills and everything breaks even.
And then summer hits, the temps spike, there are brownouts in California, and electricity rates spike. Suddenly the utility payment is three times expected. DOH!
Okay, so each person has been contributing a set extra amount through the winter months to build up a surplus. That surplus can then be used in the summer when the electricity rates go up. Brilliant, still pay as you go, no ponzi.
Except Eddie, who handles the bills for the group, decides he’s going to spend the surplus on snacks for movie night and whatnot for the group. He’ll just put in an IOU, and then take it from money collected some other time for snacks in the snack fund. The snack fund nobody actually sets up.
Then summer rolls around, and the utility bills spike, and the surplus fund has been spent on snacks, and there isn’t a snack fund, and now Eddie has to hit everyone up for an extra contribution to pay back the IOU.
It’s not really a Ponzi scheme, it’s just bad bookkeeping.
Extending the analogy, Eddie decides instead of hitting everyone up for an extra payback for the IOU, he’ll just pay it out of his pocket/savings, then pay himself back the next couple months from the surplus. Still not Ponzi, just really bad bookkeeping.
Ponzi scheme:
Eddie and Tommy decide to get an apartment together. They decide to share bills. Eddie collects some from both of them, but it isn’t enough to cover the rent and utilities. So Eddie asks Johnny to move in, and Johnny has to contribute. He uses Johnny’s pay in to pay off last month’s bills. Utilities go up slightly, and they need more money, so Eddie asks Dave to move in and contribute, so he uses Dave’s payment to pay off the previous month’s bills. The landlord catches wind that there are now 4 people in the apartment, and makes them change the rental agreement/get a second apartment. Now their rent doubled, but the contributions haven’t gone up. So Eddie goes off to find some other suckers to move in and share expenses, until the landlord catches on and tells them to get another apartment. Lather, rinse, repeat.
No person ever really contributes his full share, the money to pay off this month’s bills comes from the new guy moving in, and that only lasts until the landlord makes them pay more because there are more people living there. If they could put 4 people into one apartment, maybe the same contribution would cover the costs, but because they are only allowed 2 to an apartment, every time they increase the people, they have to increase the rent.