My son did an interesting experiment with a can of soda. Weigh the can before opening. Open the can and weigh it again. Wait a while (for the soda to go flat) and weigh it again I don't understand the result he got but here's what he saw: Immediately after opening the soda, the weight of the can (and soda) went up. (0.2g) Once the soda went flat, the weight went had gone down (after one hour) (0.2g) I put the results down to experiemental error, so I repeated his experiment and get the same results. Any ideas what effect we're observing? I'm surprised that the CO2 lost from a can going flat would have sufficient mass to cause any effect, and I definitely don't know why it would go UP in mass.