As a class or in a group make a list of things that in your opinion are IMPOSSIBLE.

Some of the things my class came up with:

Live on the sun
Get a 10 in Biology
Suck your elbow
Live forever
Be underwater for a long time
Run faster than 10km/h
That Jesús (student in class) won't eat pizza on a Friday night.

With this list on the board I asked them to think which ones they thought were not actually impossible and explain how they could be done.

For example study harder in biology, cut off your arm and suck your elbow, fly in a plane, make some robot legs that will help you run that fast and so on...

Secondly I asked them to decide which one from the list they thought was the most impossible.

other ideas:
Rank the list (maybe a smaller list) from impossible to possible