When Apple updates the Mac operating system it often borrows features from iOS - and Siri was one of these.
Siri arrived on the Mac with the MacOS Sierra update in September 2016.
There are three ways to activate Siri on the Mac:
1) You can access Siri in the Dock at the bottom of your screen (it sits between the icons for the Finder and Launchpad)
2) You can find Siri in the Menu bar at the top of the screen (look for the purple and blue logo next to the magnifying glass search icon).
3) You can activate Siri by pressing the command key and space bar for a few seconds.
There is also a software button in the Control Strip in the new MacBook Pro's Touch Bar (reviewed here).
IF you are running MacOS Sierra and Siri isn't working for you it may be that you skipped set up during the installation of Sierra. In that case go to System Preferences > Siri and click the box beside Enable Siri.
Now Siri is up and running you can ask some questions:
For example, after clicking on the Siri icon, you can say: "Get me directions to London" or "What's the weather like today".
You can use Siri to change system settings, for example, say "increase brightness" and Siri will brighten up your display for you. Or "turn on bluetooth" to switch on the bluetooth chip, something that saves a lot of time that would have been spent going though System Preferences menus. Try asking "how much space is left" to find out how much storage you still have available.
Unfortunately Siri can't tell you "how many gigabytes of photos do I have".
Siri on the Mac is supposed to remember what you just asked when you ask another question - so in theory you should be able to ask What's the weather like in London, followed by how do i get there, except in our tests it doesn't work. When we asked how do I get there Siri just showed us the weather in our home town like it was trying to tell us not to bother.
Another disappointment was that Siri couldn't play particular tracks, denying that they were in our Music. You should also be able to ask Siri to 'FaceTime Dad", except we were told we needed to continue in the app.
A handy Siri tip: if Siri has misheard you just click on the text where it has attempted to transcribe your words and correct them there before pressing return. You can also use this method to search using Siri without actually saying anything! Well you might need to say one word to get started, but then you can select that word and type in your question - handy if you don't want the person sitting next to you at work to know you were searching for film times for the Lego Batman movie.
OF course you don't have to use Siri to search your Mac, you can still use good old Spotlight (activated by quickly pressing Command and Space). You can even 'speak' to Spotlight in the same sort of way you would Siri since it got 'natural language' search in El Capitan which means you can construct your search query in a more colloquial way, phrasing it in the sort of language you'd use in everyday conversation. For example: "documents I wrote in July" or "photos added today".
Related: Complete guide to Siri for Mac Plus: How to fix Siri problems and Spotlight tips
Siri has a huge range of commands, covering a wide range of applications and functions.
Here's a taste of what Siri can do on the Mac:
"Show me photos I took in France"
"Mute the volume"
"Tweet I love Macworld" [you can add a hashtag on the end, too: just say "hashtag Macworld is the best" or whatever]
"Have I got any emails from Mandie?"
"How do you spell broccoli?"
"Do not disturb"
"Show me files I worked on yesterday”
"Post on Facebook"
Not everything you can ask Siri on the iPad and iPhone currently applies on the Mac, but a lot of it does. So for some humorous queries, take a look at Funny things to ask Siri.