What are the possible ways to scratch in pool?

"Scratching"? means that you either hit the cue ball into a pocket, your opponent's ball into a pocket, or you didn't hit any balls at all. When you scratch, you automatically lose your turn.

I have found three ways to "scratch":

1)The cue ball strikes the 8-ball first (unless it is the only legal shot).
2)The cue ball strikes one of your opponent's balls first.
3)The cue ball does not strike a rail or a legal ball.

Here is a helpful guide:

That is in 8-ball, the game you play with all 16 balls. There are different ways to scratch when playing 9-ball or any variation of that.

You will be assessed a penalty if:
• your cue ball fails to hit any other ball
• the first ball your cue ball hits is not the target ball
• you scratch (that is, pocket the cue ball)
• on a shot in which no ball is pocketed, no ball hits a railing
following the initial collision of the cue ball and the target ball

It has the same rules as 8-ball and more as 9-ball requries you to hit the balls sequentially. Plus the penalty of scratching in 9-ball is much more severe as your opponent can move the cue ball anywhere on the table, in 8-ball it is limited to a certain side of the table.

It is also a scratch if you knock the cue ball off the table.