Here's why:
* Adjective Clause: An adjective clause modifies a noun or pronoun.
* "That": This is a relative pronoun that introduces the clause and connects it to the noun "guy".
* "Bought me a soda": This is the rest of the clause, describing the action the guy performed.
The complete adjective clause tells us what kind of guy we're talking about - the one who bought the speaker a soda.