How do you play Scrapheap Scrounger's ability?

Question

Scrapheap Scrounger has an ability that allows you to return it from the graveyard to the battlefield. However, it doesn't say you can play this ability while the card is in the graveyard. I am trying to figure out the order of effects that would let me use this.

I assume that the idea is you can play it in response an effect that would destroy it or otherwise send it to the graveyard. But, wouldn't the Scrounger's ability then resolve first, at which point the card is still a creature on the battlefield and not a card in the graveyard? Then whatever effect is about to kill it would resolve and kill it? Doesn't that even apply to dying from attack damage?

Answer 1

Scrapheap Scrounger's activated ability works if and only if it's in the graveyard. That is implicit for all such abilities, when they wouldnt make sense otherwise.

Generally, you are correct in that abilities on permanents only function on the battlefield. There are, however, exceptions to that. The Scrounger's ability falls under one of them:

112.6. Abilities of an instant or sorcery spell usually function only while that object is on the stack. Abilities of all other objects usually function only while that object is on the battlefield. The exceptions are as follows:

112.6k An ability whose cost or effect specifies that it moves the object it’s on out of a particular zone functions only in that zone, unless that ability’s trigger condition, or a previous part of that ability’s cost or effect, specifies that the object is put into that zone. The same is true if the effect of that ability creates a delayed triggered ability whose effect moves the object out of a particular zone.

Answer 2

Just to complement Hackworth's answer explaining how this actually works (you activate the ability only when Scrapheap Scrounger is in the graveyard), I thought it might be helpful to point out how an ability that works roughly how you describe might be written.

The closest thing is probably regeneration. For example, Drudge Skeletons says:

{B}: Regenerate Drudge Skeletons. (The next time this creature would be destroyed this turn, it isn't. Instead tap it, remove all damage from it, and remove it from combat.)

You activate the ability before it dies, and it creates a "regeneration shield" so that when it would die, it instead gets to stick around as described. Applying the regeneration shield is an example of a replacement effect. In the end this is sort of similar to what you describe, in that it lets you pay mana, perhaps in response to something that would kill the creature, in order to prevent a creature from dying.

There are also other kinds of replacement effects that can prevent creatures from dying, like Necromancer's Magemark:

[...] If a creature you control that's enchanted would die, return it to its owner's hand instead.

The words to note there are "would" and "instead", which indicate a replacement effect. This time there's no mana payment involved, but it still does prevent a creature from dying. This is a pretty rare effect, though; it's far more common to see dying replaced with something even worse: "If a creature dealt damage this way would die this turn, exile it instead."