Here's why:

* Reactivity Series: Metals higher in the reactivity series than hydrogen can displace hydrogen from acids.

* Reaction: The reaction between a metal and hydrochloric acid produces a metal chloride salt and hydrogen gas:

* Metal + Hydrochloric acid → Metal chloride + Hydrogen gas

* For example, zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas:

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

Note: Not all acids will react with all metals to produce hydrogen gas. The reactivity of the metal and the strength of the acid are important factors.