Here's why:

* Arrhenius Definition of Acids: A common definition of acids is that they produce hydrogen ions (H+) when dissolved in water.

* Examples:

* Hydrochloric acid (HCl) -> H+ + Cl-

* Sulfuric acid (H2SO4) -> 2H+ + SO4^2-

* Nitric acid (HNO3) -> H+ + NO3-

While not *all* acids contain hydrogen (e.g., some organic acids like acetic acid have a different structure), the vast majority of acids commonly encountered in chemistry do.