I would start class by having the students nominate a "honorary scientist". Ask the scientist to stand outside of the doorway and await further instructions. Then give the students in the classroom the opportunity to do whatever they wanted (within reason) for one minute while you go speak with the scientist. Encourage them to stretch, check their homework, talk, etc. Begin the free time as soon as you step out to speak to the scientist. Ask the scientist to continually observe the classroom from her position outside of the classroom. At about thirty seconds into this brief experiment ask her to open the door and observe approximately how many individuals stop what they are doing and turn their attention to her presence the room. Have her close the door and continue to observe for the remaining thirty seconds until you both finally enter the room together, ending the experiment.

As everyone settles down, ask the scientist to try to sum up her observations of the class's behavior in one word. Then ask her to rank on a scale from 1-10 to what extent this behavior was exhibited over the time of her observations. She may say that the class was chaotic and rank it as a constant 7 over the entire duration. Each student would then have contributed to this state of chaos to varying extents but they in all resulted in a chaotic 7. Link this to the notion that a system can have a temperature quantity associated with it that is just like the activity of the class. Similarly mass is a quantity that can be associated with the number of students present in the classroom. It is a quantity (like temperature) that can be associated with the internal state of a system.

Then explain to them that weight is like the attraction of their attention to the scientist when she first opened the door. Some people will surely have turned their attention to her, but more will have turned their attention to the both of you. The lesson will be that while the amount of students stayed the same in both cases, attention/attraction was greater when the two of you walked in.