Physical lock

If you ever study computer science, you will know the importance of a "lock". If there is concurrent access to a piece of data, you will probably need a "lock", or else you will be run into trouble.

Okay, let's move to a more realistic example.

A few days ago, there was a broken light bulb inside my home, so it's natural to buy a replacement one, right?

But due to the busy schedule, I didn't do so immediately. And someone else was buying that for me, nice thing, right?

However, no one ever told me that a new bulb is already in place. Guess what, I bought one on the same day without knowing this fact!

Therefore, it's funny enough and so now we ended up to have several of them at home. It's a good example of the need of a "lock", or to make it simple, it's the importance of communications.