Here's why:

* Metals are good reducing agents: They readily lose electrons to form positive ions.

* Acids are good oxidizing agents: They readily gain electrons.

When a metal reacts with an acid, the metal donates electrons to the hydrogen ions (H+) in the acid. This reduces the hydrogen ions to hydrogen gas (H2), which bubbles out of the solution.

Here's a simplified chemical equation:

Metal + Acid → Salt + Hydrogen gas

Example:

Zinc (Zn) reacts with hydrochloric acid (HCl) to produce zinc chloride (ZnCl2) and hydrogen gas (H2):

Zn + 2HCl → ZnCl2 + H2

Important Notes:

* Not all metals react with all acids. The reactivity of metals varies.

* Some metals, like copper and silver, are less reactive and require stronger acids to produce hydrogen.

* Other substances besides metals can react with acids to produce hydrogen, but these reactions are not as common.