How do I lock phone phone with OK google?

Unlocking a phone with okay google is useful. However, how can I lock it again using okay google. It's very handy being able to talk to my phone whilst working on the laptop but in the interests of battery power locking it again would help. Particularly if I'm using bluetooth and the phone is in my pocket. Time out is set to 15 seconds but a lot can happen in 15 seconds.