Viewing slits and fuel cell: what they have in common

1. Step 1. Viewing slits — is part of — Zoetrope. Slits show the frames one at a time.
2. Step 2. Zoetrope — is made of — Paper. The pictures are on a paper strip.
3. Step 3. Paper — is material for — Hot air balloon. It was lined with paper on the inside.
4. Step 4. Hot air balloon — evolved into — Airship. A steerable balloon.
5. Step 5. Airship — uses — Hydrogen. Zeppelins were filled with hydrogen.
6. Step 6. Hydrogen — is used in — Fuel cell. The fuel is hydrogen.

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