On an iPhone or iPad, Siri can be activated by holding the Home button on compatible models or holding the Side button on devices without a Home button. On the Mac, you can click on the Siri app icon on the dock or the menu bar, or press and hold the command key and the space bar. On a Mac with a Touch Bar, you can press the Siri icon on the Touch Bar. On 2018 MacBook Air and Pro models or the iMac Pro, you can activate Siri with a "Hey Siri" command.
