On Saturday afternoon, horse racing fans will tune in for “the most exciting two minutes in sports” at Churchill Downs in Louisville, Kentucky. This year, there will be one thing on the minds of every betting type watching the Kentucky Derby: mud.

When it comes to calculating the odds of which horse will win the race, there are a lot of factors that affect the odds. Among them, Saturday’s weather has grown to be a huge contender. If it rains — as it’s predicted to — the track will be muddy, and how each horse performs could be greatly affected by this. This has gotten experts analyzing everything from the amount of sand in the Churchill Downs track — there is a lot apparently, which drains water well — to each horse’s finishing record in rainy races.

Weather aside, there are a lot of other factors that can contribute to a bettor’s opinion on a horse’s chances of winning. They include its racing record leading up to the Derby, the horse’s pedigree, and even a bettor’s opinion on the physical caliber of the horse itself.

For the uninitiated, betting odds merely present the likeliness that an event will happen, in this case that a horse will win a race. For example, Saturday morning’s odds on Always Dreaming were 4-1. These are fractional odds. So the probability equation here is: 1 / (4 + 1) = 0.20.

A more universal way of explaining this is with letters. If 4/1 becomes A/B, the calculation is: Probability (%) = B / (A+B).

The official odds are announced by the track staff based on their own personal research. You can also peruse the betting odds being offered from a plethora of online sources to help you determine the chances that a horse is going to win, and a slew of variables can mess with these probabilities up until the hour of the race. But it wouldn’t be any fun if this was totally predictable, would it?

The Kentucky Derby post time is 6:34 p.m. Eastern, you can watch coverage on NBC starting at 2:30 p.m., or stream it live on NBC Sports Live.

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See also: A.I. That Predicted Kentucky Derby Superfecta Reveals 2017 Bets

Photos via Getty Images / Michael Reaves