General anaesthesia and joystick: what they have in common

1. Step 1. General anaesthesia — uses — Chemical reaction. Ether and chloroform are chemical agents.
2. Step 2. Chemical reaction — is used in — Percussion cap. Mercury fulminate explodes when struck.
3. Step 3. Percussion cap — evolved from — Flintlock. The percussion lock replaced the flintlock.
4. Step 4. Flintlock — has part — Spring. A mainspring.
5. Step 5. Spring — is part of — Joystick. They return the stick to the centre.

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