How to take a snapshot of snapchat?

I'm on an iPhone 4S, and I know that to take a snapshot u push the home and lock button, but whenever I do it with the snapchat app, it never works and just goes back to the message list screen instead of taking a snapshot. Is there a trick to this? Help?

Added (1). I know it's possible because my friends have done it, even on my phone, but I can't seem to get it

You take it like a regular snapshot. It's tough but that's how you do it

The app is modified it so you can't snap shot it. Maybe just for protecting others? You could send a really embarrassing photo but no one could snap shot it. All for the sake of privacy I suppose