Final answer:
Two different instruments playing the same note sound different because of timbre, which refers to a sound's purity and is influenced by frequency, amplitude, and timing of sound waves.
Step-by-step explanation:
The reason two different instruments playing the same note sound different is due to the concept of timbre. Timbre refers to a sound's purity and is affected by the complex interplay of frequency, amplitude, and timing of sound waves. Each instrument produces a unique combination of frequencies and intensities, resulting in a distinct timbre. For example, a piano playing middle C will sound different from a violin playing the same note.