If you put a little bit of water in the tin and put the lid on, it would pop up because the water becomes steam from the heat on the gas oven. There were no holes to let the steam escape so the steam pushed the lid off the tin can. Some of the water disappeared because it heated up, turn into steam and then disappeared when the lid popped up.
When the lid popped up every body got a shock. It was funny to watch the video.
