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Not sure what you mean by "sitting down." You can create location based Reminders in the Reminder app.
  • Add a location: Tap Location, then choose where you want to be reminded—for example, when you arrive home or get into a car with a Bluetooth connection to your Apple Watch.
 
Not sure what you mean by "sitting down." You can create location based Reminders in the Reminder app.
  • Add a location: Tap Location, then choose where you want to be reminded—for example, when you arrive home or get into a car with a Bluetooth connection to your Apple Watch.

Sitting down in a chair or couch.
 
Sitting down in a chair or couch.
Well, all I know is you can set reminders/alerts when you arrive/leave a location. I don't know of any way to set reminders when you are doing a particular activity.
 
I saw this post on Reddit, not sure what the sitting down has to do with it. But yes you can easily tell Siri to set a reminder when you arrive at a location.
 
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