Don't make the server remember anything.

Instead of the server keeping track of who you are, your phone or browser carries an ID card (like a cookie). You show this ID card with every single click. Because the server remembers nothing, you can add 1,000 new servers instantly.

Never change data, only make copies.

If two parts of a program try to change the same word at the same time, it crashes. Functional programming solves this by making a rule: you can never erase or change data. You just make a brand new copy with the changes. This lets computers run safely at super speeds.

Give everyone their own mailbox.

Instead of all parts of a program sharing a giant memory pool, we break the program into tiny "actors". Each actor works totally alone and only talks to others by sending text messages. If one actor crashes, the others keep working perfectly.

Write down history, don't erase it.

Instead of having a bank account that says "$50", the computer writes a diary: "Added $100. Spent $50." To know your total, it reads the diary from the start. This is incredibly fast to write down. To read fast, we create a totally separate "fast lane" just for viewing the totals.