Here's why:

* pH scale: The pH scale measures the acidity or alkalinity of a substance. It ranges from 0 to 14, with:

* 0-6: Acidic

* 7: Neutral (like pure water)

* 8-14: Alkaline (or basic)

* Sour taste: The sour taste we perceive is caused by the presence of hydrogen ions (H+) in the substance. The more hydrogen ions there are, the lower the pH and the more acidic the substance.

Examples of sour substances:

* Lemon juice (pH around 2)

* Vinegar (pH around 3)

* Sour milk (pH around 4)

So, if something tastes sour, it's a good indication that it's acidic and has a low pH.