Here's why:

* Properties: Sulfur is a brittle solid at room temperature, a poor conductor of heat and electricity, and has a low melting point. These are typical characteristics of nonmetals.

* Chemical Behavior: Sulfur tends to gain electrons to form anions (negatively charged ions), which is common for nonmetals. It also readily forms covalent bonds with other nonmetals.

* Location on the Periodic Table: Sulfur is located in Group 16 (VIA) of the periodic table, which primarily consists of nonmetals.

While some elements near the "staircase" line separating metals and nonmetals exhibit some properties of both, sulfur's overall behavior clearly classifies it as a nonmetal.