Why does the tails is not straight?

Please provide more context! I need to know what "tails" you are referring to.

For example, are you talking about:

* A dog's tail? This could be due to a variety of reasons, including breed characteristics, injury, or health conditions.

* A coin's tail? This is likely a design choice.

* A comet's tail? This is due to the force of the sun's radiation pushing away dust and gas.

Once you tell me what "tails" you're talking about, I can give you a more specific answer!