CC BY-SA: the 'viral' share-alike rule

ShareAlike (SA) forces anyone who remixes, tweaks, or builds on your work to release their version under the identical licence. This is the 'viral' clause: it prevents someone from taking a CC BY-SA image, adding their own content, and then slapping a more restrictive 'All Rights Reserved' label on the whole thing. Contrast this with CC BY, where adaptations can use any licence as long as the original creator is credited.

A common trick in EPSO questions is to present a composite work that mixes a CC BY-SA photo with a CC BY-NC-ND video. Because the SA element demands the same licence for the whole adaptation, the more restrictive ND (no derivatives) or NC (non-commercial) terms clash and make the composite impossible to license legally. To eliminate wrong answers, remember that SA only applies to adaptations, not to mere collections: if you just put a CC BY-SA image alongside other works without altering them, the SA rule does not kick in.

To test yourself, ask: 'If I modify a CC BY-SA song and add my own lyrics, what licence must the new song carry?' The answer is CC BY-SA, no exceptions. This single rule is the core of the SA element, and the exam will try to trick you by offering a different licence or claiming you can choose any Creative Commons option.