We call this "roll-over", when no-one wins so the pot accumulates. This week's draw doesn't need to be profitable, the overall scheme needs to be profitable.

Say $2MM in tickets are sold, putting $1MM in the pot. And no-one wins. So next week, $2MM in tickets are sold, putting $1MM in the pot - which already contained $1MM from last week. Continue this over a few weeks.

After 5 weeks you have $5MM in the pot - and another $5MM in profits. On week 5, $2MM in tickets could buy $5MM in winnings, but that doesn't mean the scheme is negative. It just means this week's draw looks like a good deal.

States love lotteries because the house cannot lose - their share is taken before it goes in the pot, so the pot is never a liability.

They're not, generally, but if the stars align correctly, nobody will win the jackpot until it's gotten so large that buying every possible combination will - if you're the only winner - pay you more than the base odds. But then you're betting that you will have the only winning ticket.

I'll usually buy a few tickets if it hits that point. Cheap fantasy.

They’re progressive jackpots, the prize pool increases until someone wins, at which point it resets. So a ticket for an individual drawing can be +EV if no one has won in a while, with the excess money coming from players in previous games rather than the lottery administrator.

The recent Powerball lottery prize was for $1 billion but there are fewer than 300,000,000 possible tickets. But like somebody else mentioned, there's always the possibility of multiple winners.

For the recent billion dollar prize, a single person won.

The prizes get huge because when there's no winner, the prize rolls over to the next drawing.

There is a fixed probability of matching all numbers but the value of doing so varies, so sometimes the EV can be positive.