The funny thing is your machine does respond!

I think it would be hard to explain every detail... but basically your computer does communicate with others. If a service has a bug (and software has many bugs, the more complicated, the more bugs has it following experience or Murphy's Law) then you can missuse this part of the software. You would wonder how many services or vulnerabities a normal computer offers. If you interested learn more about internet and operating system, protocolls, services... think geek!