Here's why:

* Acids contain hydrogen ions (H⁺).

* Metals are good electron donors.

* When a metal reacts with an acid, the metal atoms donate electrons to the hydrogen ions from the acid.

* This process reduces the hydrogen ions to hydrogen gas (H₂), which is released as bubbles.

Example:

The reaction between zinc (Zn) and hydrochloric acid (HCl):

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

In this reaction, zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas.