Why can't double-replacement reactions start with one reactant is a pure metal?

Here's what I get

Explanation:

In a double displacement reaction, two metals exchange partners:

ZnCl₂ + 2AgNO₃ ⟶ 2AgCl + Zn (NO₃) ₂

You can have a displacement reaction that starts with one pure metal:

Zn + 2AgNO₃ ⟶ 2Ag + Zn (NO₃) ₂

In this case, however, one metal is taking the other's partner, so we call it a single displacement reaction.