Did you know that you can now activate Siri by simply saying “Hey Siri” while your iPhone is plugged in? To enable this feature, just go into Settings, General, Siri and ensure “Allow Hey Siri” is toggled on.
The feature was first introduced in iOS 8 last fall but has been improved through each update, and with recent versions you can now even simply state your whole request without pausing after the opening line – for example, “Hey Siri wake me up in 15 minutes.”
The only catch is that your iPhone has to be connected to power for this to work, which is probably a reasonable compromise both in terms of preserving battery life (since your iPhone has to keep the mic on and listening for the phrase all the time), and also limiting instances of accidental activation.
Should Siri ever wake up accidentally, or if you change your mind, you can also dismiss Siri with a phrase such as “never mind” or “go away.”.