There are several lists of "improv rules" to be found on the internet - because we get so many questions about Rules of Improv we are listing a few here; if known we list the origin (let us know if you know more).
We are not necessarily endorsing these rules; your mileage may vary. And never forget: it`s good to know and understand rules (some rules) but once you have mastered them, by all means feel free to break them. You may also want to read Improvise - Scene from the inside out for a discussion why rules in Improv may not be all that relevant.
List of Improv Rules
Failure is OK - which is the most famous space shuttle? -- paraphrasing Keith Johnstone