In computer science, simple "Yes/No" questions are called decision problems. A decision problem is called decidable if we can write a program that can always find an answer for such a problem.

An example for such a decision problem is "does this program contain a computer virus?". If this problem was decidable, there could be another program that could always correctly answer this question with "yes" or "no". A mathematician named Henry Gordon Rice showed that such decision problems about the behavior of another program are always undecidable, unless the answer is always the same.

As most viruses are hidden inside other software and don't always expose their true intentions, this decision problem can't be solved according to Rice's theorem.