DES MOINES, Iowa >> There is no mystery why the Mega Millions jackpot has grown to $1.55 billion, making it the third-largest ever ahead of Tuesday night’s drawing.

The prize has ballooned because no one has matched the game’s six winning numbers since April 18, amounting to 31 straight drawings without a big winner. The nearly four-month-long unlucky streak could be all the sweeter for the person who finally lands the top prize, which is inching toward the record lottery jackpot of $2.04 billion won in 2022 by a player in California.

“It’s a fun thing,” said Merlin Smith, a retired real estate appraiser who stopped Monday at a gasoline station in Minneapolis to buy five tickets. “But if you’re depending on winning, you’d be disappointed a lot.”

WHY DOES IT TAKE SO LONG FOR SOMEONE TO WIN?

It has been a long stretch of jackpot futility, but Tuesday night’s 32nd straight drawing since the last winner still isn’t a record. The longest run for a Mega Millions jackpot was 36 drawings that ended on Jan. 22, 2021, with someone winning a $1.05 billion jackpot. The record number of lottery draws was for a Powerball prize that ended after 41 drawings when someone won the record $2.04 billion jackpot.

Wins are so rare because the odds are so miserable, at 1 in 302.6 million. When a drawing fails to produce a big winner, the prizes roll over for weeks. Bigger prizes sell more lottery tickets, which also drives more revenue for the state services lotteries fund.

HOW MANY NUMBER COMBINATIONS ARE THERE?

There are roughly 302.6 million possible number combinations for the five white balls and separate gold Mega Ball in Mega Millions. The white balls are numbered from 1 to 70 and the Mega Ball goes from 1 to 25.

To put that number in perspective, consider that all the tickets sold for last Friday’s drawing produced only about 35% of the possible number combinations. That means about 65% of possible combinations — or nearly 200 million options — were not covered. Lottery officials expect that as sales increase ahead of Tuesday night’s drawing, the potential combinations covered will rise to just over 41%.

DON’T EXPECT TO PUT $1.55 BILLION IN THE BANK

Yes, the money will come pouring in if you win the Mega Millions jackpot, but don’t expect a $1.55 billion check to pop into your bank account.

That’s because the estimated $1.55 billion prize is for a sole winner who chooses to be paid over 30 years through an annuity. Jackpot winners almost always choose a lump sum payment, which for Tuesday night’s drawing would be an estimated $757.2 million.

For either prize option, a big slice of the money would go toward federal and possibly state taxes.

State lotteries typically lop off 24% of winnings for federal taxes, and the bill can run even higher because the top federal income tax rate is 37%. Many states also tax lottery winnings.

As more people buy tickets, the chances also increase that more than one person could match all six numbers. For example, a $1.586 billion Powerball prize was won in 2016 by three players in California, Florida and Tennessee. That means a winner could end up with only a portion of a very large jackpot.

WHAT IS THE BEST WAY TO CHOOSE MY NUMBERS?

Players overwhelmingly choose the easy pick option when buying tickets, letting the machine generate numbers for them. In Iowa, for example, more than 90% of Mega Millions purchases were to people who let the machine choose, rather than selecting the numbers themselves.

The odds are the same no matter if the machine chooses the numbers, or you do.

WHERE IS THE LOTTERY PLAYED?

Mega Millions is played in 45 states, Washington, D.C., and the U.S. Virgin Islands. The game is not played in Alabama, Alaska, Hawaii, Nevada and Utah.