Only two teams in modern NFL history have put together perfect regular seasons. Only one has run the table through the playoffs, and that was nearly a half-century ago.
While the 5-0 Kansas City Chiefs and 5-0 Los Angeles Rams are 31 percent of the way to regular-season perfection, history suggests they have their work cut out for them.
In the 48 completed seasons since the 1970 AFL-NFL merger, 80 teams have started 5-0. Including the playoffs, 79 have eventually lost.
The Chiefs also started 5-0 last year, five years ago (2013) and 15 years ago (2003). They wound up losing a combined 14 regular-season games in those three campaigns.
This is the eighth time the Rams have started 5-0 since the merger. They wound up losing at least two games in each of the previous seven campaigns.
Will either team go 16-0 this year, or better yet, 19-0? Let's break it down.
The Chiefs will have to fight for 6-0, let alone 16-0
Week 6: @ Patriots -- KC has won back-to-back games vs. New England.
Week 7: Bengals -- The Bengals are off to a 4-1 start.
Week 8: Broncos -- Denver has lost three straight after a 2-0 start.
Week 9: @ Browns -- Cleveland has yet to lose at home.
Week 10: Cardinals -- Arizona is the league's lowest-scoring team.
Week 11: @ Rams -- Los Angeles is also unbeaten.
Week 12: BYE WEEK
Week 13: @ Raiders -- Oakland beat Kansas City last October.
Week 14: Ravens -- Baltimore has lost two of its first three road games.
Week 15: Chargers -- KC has won nine straight against the Chargers.
Week 16: @ Seahawks -- Seattle nearly beat the Rams at home in Week 5.
Week 17: Raiders -- KC has won five straight home games versus Oakland.
There won't be two 16-0 teams this year, because the Chiefs' biggest regular-season challenge will come in their Week 11 Monday Night Football matchup with the Rams at Estadio Azteca in Mexico City.
That'll be a tough trip to a high altitude for the Chiefs, but they may suffer their first loss well before that contest.
They're early 3.5-point underdogs for their Week 6 prime-time matchup with the New England Patriots at Foxborough, according to OddsShark. While they scored a combined 83 points in two victories over the Patriots in 2014 and 2017, respectively, those wins came with veteran Alex Smith under center.
New franchise quarterback Patrick Mahomes has a higher ceiling than Smith and is off to an MVP-caliber start, but he's also a gunslinger who figures to be prone to off days. He had a less-than-awesome outing against the Jacksonville Jaguars on Sunday, but a mistake-prone Jags offense bailed him out.
Will the seemingly rejuvenated Patriots be as forgiving? What about the 4-1 Cincinnati Bengals or the better-than-their-record Cleveland Browns, both of whom are on the schedule within the next four weeks as well? That Browns game is in Cleveland, where Hue Jackson's team has yet to lose in three outings this season.
Even if the Chiefs survive that stretch and come out of their Week 12 bye with an 11-0 record, they have tough matchups with the Baltimore Ravens, Los Angeles Chargers and Seattle Seahawks (on the road) awaiting in the final month of the season. They have three potential gimmes remaining (home matchups with the Denver Broncos, Arizona Cardinals and Oakland Raiders), but everything else should be fairly challenging for a team that has surrendered an alarming 25.8 points per game.
Kansas City is also a notoriously streaky team. The Chiefs were 5-0 at this point last season, but they then lost six of seven before reeling off four straight wins to finish the 2017 campaign. They won eight of nine at one point in 2016, and they had both a 10-game winning streak and a five-game losing streak in 2015.
To run the table this year, they'd have to win 20 consecutive regular-season games dating back to last season. With a young quarterback and a below-average defense, that's extremely unlikely.
The Rams have a slightly clearer path, but don't count on it
Week 6: @ Broncos -- Denver nearly beat KC at home in Week 4.
Week 7: @ 49ers -- Los Angeles won 41-39 in San Francisco last season.
Week 8: Packers -- Green Bay has yet to win on the road.
Week 9: @ Saints -- The Rams beat the Saints in L.A. last year.
Week 10: Seahawks -- Seattle won in L.A. last October.
Week 11: Chiefs -- Kansas City is also unbeaten.
Week 12: BYE WEEK
Week 13: @ Lions -- Lions have beaten the Packers and Pats at home.
Week 14: @ Bears -- Chicago is 2-0 at home.
Week 15: Eagles -- Philly beat the Rams in L.A. last December.
Week 16: @ Cardinals -- L.A. crushed Arizona in their last three meetings.
Week 17: 49ers -- L.A. lost to San Francisco in Week 17 last season.
The Rams have the benefit of playing in a cakewalk division. The shell-of-their-former-selves Seahawks are 2-3, the depleted San Francisco 49ers are 1-4 and are missing their franchise quarterback, and the rebuilding Cardinals are also 1-4 while averaging only 13 points per game.
Four of L.A.'s final 11 games come against those NFC West foes. At least three of those four should be relatively easy wins.
However, six of the Rams' final 11 outings come on the road, including the only two games in which they might not be favored: a Week 9 matchup with the New Orleans Saints at the Superdome and a Week 14 matchup with the upstart Chicago Bears in potentially cold weather at Soldier Field.
In addition, they host the increasingly desperate Green Bay Packers (Week 8), the also-perfect Chiefs (Week 11) and the defending Super Bowl champion Philadelphia Eagles (Week 15). They could have trouble in a Week 10 matchup with a Seattle team that put up a fight in their Week 5 meeting as well.
Even if they beat the slumping Broncos and downtrodden 49ers on the road over the next two weeks to move to 7-0, the Rams might not have another gimme until they host San Francisco in Week 17. If they're 15-0 at that point, there'll be an entirely new debate about resting key players.
Head coach Sean McVay hasn't shied away from doing so early in his tenure with the Rams. He rested his starters in a Week 17 loss to San Francisco last season, and he kept most of his best players in bubble wrap this preseason.
Per Jimmy Kempski of PhillyVoice, the Rams entered this season as the fourth-youngest team in the NFL. On one hand, that could make it easier for them to outlast tired opponents. On the other hand, it could make them susceptible to a bad day at the office.
Like Mahomes, quarterback Jared Goff is only 23 years old. He's likely to have another shaky performance or two this fall. Even if he doesn't, a defense that has surrendered 62 points over the last two weeks might eventually leave the offense with an impossible margin for error.
Considering the non-divisional schedule, the potential for the injury bug to hit (it already has to an extent), their lack of experience, their vulnerabilities on defense and a potentially apathetic mentality regarding a 16-0 regular season, it's highly unlikely the Rams win more than 13 or 14 games, let alone 15 or 16.
In other words, members of the 1972 Miami Dolphins figure to soon have another chance to toast the only completely perfect season in modern NFL history, even if that tradition is merely urban legend.
Brad Gagnon has covered the NFL for Bleacher Report since 2012.