Matter in a neon sign

PLASMA

Hint: This state of matter is often found glowing in a tube, and it's not a solid, liquid, or gas. (This hint guides the solver by narrowing down the possibilities—it’s a state of matter, not the usual three, and is associated with glowing light, which is key to neon signs.)

The clue uses a double meaning of "matter." In a scientific context, "plasma" is a state of matter (alongside solids, liquids, and gases), often found in neon signs where gas is ionized to produce light. The wordplay hinges on interpreting "matter" both as a physical substance and as a reference to the glowing, ionized gas inside a neon sign. Thus, "plasma" fits perfectly as the answer.