Very well. Let's say you (or other teachers) have left worksheets, review, vocabulary lists, or other work the students can complete on their own. I'm assuming your reaction to the sub looking at Facebook during this time would be to inform the regular teacher - and perhaps the P - that it might be best if a different sub is called next time. Ah, but that's the biggest problem I see with these responses; how many people actually spend 30 seconds or less checking messages, Facebook, AtoZ or any other sites? You can check your text messages fairly quickly, but then you see one (or more) you feel needs a response, so you spend a few more "seconds" texting a reply. Then you may find another one that needs an answer, etc. Then you might decide to check FB, see a couple of cute pics you want to "Like" and maybe a post by a friend You want to post a quick comment about. These activities may not take very long individually, but the amount of time adds up in a hurry when you combine them and I wouldn't be surprised if people spend a lot more time on these things than they realize.