If students are cooperative and well behaved, I find that it is useful to have them first explore a gallery by themselves, trying to figure out what is going on in the room. Then, I ask them to stand in front of their favorites and those are the ones where we spend our time.
Having them sketch in front of Hercules seems to work well.
Having them identify different things on the time line that they find interesting also works well.
I think it is important to be very firm if students are goofing around, not paying attention or wandering off.
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