Question about MacBook Air and External Monitor

Thanks for reading this. Here is my situation:

I've always owned Mac desktops but I started a new job and my company gave me a Macbook Air to use for work. It's the latest 13" model.

At work, I use it with an external monitor; I close the MacBook so I'm only using the external monitor for my screen.

Tonight, I brought the MacBook home. I may end having to work at home occasionally for my job.

At home I have a Mac Mini and a Dell 24" external monitor.

I disconnected the monitor from the Mac Mini and I plugged it into the MacBook Air.

The monitor is working just fine with the MacBook but I'm having one issue that I can't figure out:

When I close the MacBook Air, the external monitor goes black. I can't figure out how to configure the MacBook so that when I close it, the external monitor still displays my MacBook's screen.

I'm sure it's a quick fix but I can't figure it out. I'd greatly appreciate any help.

Thanks!