The hint lies in the sound you heard.
The design of this empty can isn’t the only thing special about it. It also has a rare kind of pull tab where you place your finger on the tab and pull it back to open it. Back then, beer always came in bottles. When they saw this mysterious can, they had no idea how to open it, but fortunately, the can had instructions on the back.
Incidentally, UCC’s canned coffee became well-known after the Expo. At the time, coffee was something that you only drank in cafes, but after people saw canned coffee on TB, orders flooded in for them. However, this is not a pull tab, but a can opener that comes with every can. The can opener has two holes in it. One is for you to drink through while the other is to let air escape from the can.
Though canned beverages have become matter-of-fact in modern day, this was a part of the vision of the future back then.