One way to do this "trick" is to warm up some sodium thiosulfate (hydrated) crystals. (It is readily available from photography supply stores -- used to be called "hypo", and it was the main constituent of the "fixer" in chemical process photography). At around 50-60 deg C they will dissolve in their own water of crystallization. The resulting solution supercools very readily. The solution will easily stay liquid down to room temperature in a moderately clean environment. It is likely that plunging the burning end of a straw into the solution will seed rapid crystallization, and turn the whole container to solid "ice" in a matter of a second or two. The whole process happens at or slightly above room temperature.
If you are very carefully observant you can tell whether what is being done is this trick or a similar one. Although the rapidly forming crystals look like ice, there is a very important difference: the crystals will fall to the bottom of the solution. If real ice were being formed it would be floating to the top as it was forming. You would only have a second, but if you were careful you could see the difference.