Made of high-tensile steel wire.

Lower strings are wrapped in copper for more mass --> lower frequency

Attached to the soundboard by metal pins called the "bridge." The bridge acts as nodes, maintaining the strings' tensions.

Made of spruce- light, stiff, and elastic, which is ideal for sound amplification.

Vibrations from strings travel through the bridge into the soundboard, making the sound louder. 

Holes in the soundboard helps the piano "breathe." Without them, vibrating air would be trapped underneath the wood, dampening the sound.

The hammers and dampers are made of wood with balls of felt at the end.

The wool is packed tightly enough to create a strong vibration, but soft enough to avoid a metallic sound when hitting the steel wires. 

Old school pianos used hide glue to hold the piano together.

It wasn't just because that was the only option, it also allows the wood to vibrate more freely, which preserves resonance. 

A concert grand piano holds 18 to 20 tons of tension from the strings. 

That's why the internal frame is cast iron instead of wood, otherwise the strings would pull the piano apart.