Siri uses Wolfram Alpha to find the answer to an amazing variety of questions.
Rather than return a list of search results in Google, Siri uses Wolfram Alpha to generate what it believes is the correct answer.
The results can be a little hit and miss, but for the most part are surprisingly accurate.
Here are some examples of the kind of questions you can ask:
Who is the Prime Minister of Australia?
When was the Brooklyn Bridge built?
How do you spell Brooklyn Bridge?
If Siri knows the answer (or thinks it knows the answer), it will displayed on screen along with voice feedback.
Siri can also answer much more complicated questions, such as:
What is the square root of 244?
If the Wolfram Alpha search doesn't return definitive results, Siri will offer a list of links and topics it considers to be relevant to your query.