https://www.scirra.com/manual/124/system-conditions
[quote:1yn1gpzj]Pick Nth instance
Pick the instance at a given place in the internal list of picked objects. This is most useful used in sub-events to act on separate instances. For example, in a "Sprite collided with Sprite" event, Pick 0th instance and Pick 1st instance can be used to act on each instance involved in the collision separately.
If all objects are currently picked, this condition can also be used to pick an object by its index ID (IID). For more information, see common features.
After picking the 0th instance (oldest), you action (destroy) will then only apply to that instance.