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The probability scale is a numbered line that can include any real number between 0 and 1. It provides the likeliness that some event could happen.

If the likeliness of an event is 0, then it's impossible for it to occur. For example, let's say you were to take all of the people named "Mark" on the planet and put them in a straight line. If you were to pick one of them at random, what is the probability that their name is "Rob"? This hypothetical event defies the set of laws we've set for our data. It's impossible for it to happen.

If the likeliness of an event is 1, then it's certain for it to occur. If you were to pick one of the Marks out of your line at random, what is the probability that their name is "Mark"? All of the people in that line are named "Mark", and so it's certain that the random person's name will also be "Mark".

Anything between 0 and 1 has the likeliness that it's not certain for it to happen, but it's also not impossible.

If the likeliness of an event is 0.5 (1/2), then it has an equal chance of both happening and not happening. If half of the Marks were to wear a blue shirt and the other half wore a green shirt, then the probability of picking a blue shirt-wearing Mark at random is 0.5. It was just as likely for you to pick a Mark wearing a green shirt.

Flipping a Coin

Let's examine what it really means to get heads while tossing a coin.

Is it impossible for you to flip heads? Of course not, it's one of the two sides of a coin, so flipping a coin surely means you can land on heads.

Is it certain for you to flip heads? Of course not, tails is also one of the two sides of a coin, so flipping that coin surely means you also have the likeliness of landing on tails.

A coin has two sides - heads and tails.
A coin flip is a completely random event. It's independent of any other factor such as the laws of physics. There are two outcomes a coin toss can have and either of them are just as likely to happen as the other. The probability of landing on heads is one of two, one possible outcome out of the two.

For this reason, we say that the probability of getting heads while tossing a coin is 4) 1/2.