SMTP and tunnelling shield: what they have in common

1. Step 1. SMTP — is used in — Email. Sending mail.
2. Step 2. Email — evolved from — Postal service. Electronic mail.
3. Step 3. Postal service — uses — Steam locomotive. Mail trains.
4. Step 4. Steam locomotive — uses — Railway track. It runs on rails.
5. Step 5. Railway track — is used in — Metro. It runs on rails.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

Open Evothings