I have a question which I'm hoping some of you clever people might be able to answer...

I kept some out of date yogurts (about 4 weeks out of date) for some messy fun and scooped it all out and kept it in a 2litre empty Coke bottle. I've done this before and the same thing happened. After a couple of weeks the yogurt starts foaming up and producing what could be carbon dioxide! I have to open the bottle every day to stop the foam turning into an exposive messy substance, which would be very funny and messy to use!

Can anyone explain to me why this is happening? (I'm not chemically minded...)

It's rotting, and producing decomposition gasses. Throw it away, before you poison yourself. The gasses will be a mix of methane and other breakdown components, so potentially explosive.