A trick question

 

Here's a question you won't be able to answer.

What is in this can?

 

There's no way you can answer this question because the can has no label.

Of course, it's not hard to open the can to find out, but that is pretty inconvenient unless you want to eat right away.

Opening up a single can is not particularly troublesome, but what if you have a whole pantry full of cans without labels? It quickly becomes impractical. Every time you prepare a meal, you are likely to go through a number of wrong cans before you find what you need.