water. i don't know what else but I'll get back to you
Flammability is defined at how easily something will burn or ignite, causing fire or combustion. The degree of difficulty required to cause the combustion of a substance is subject to quantification...
Because they react dangerously with fire. Thus, they are flammable.
No, xenon is not flammable. It's an inert or noble gas, and it is most reluctant to react with anything else. It won't burn.
See http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc00/icsc0013.htm