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So once we have all five of these in play, what can we do? How about making infinite mana, for starters? Tap and sacrifice Mox Opal to give yourself 3 mana, one of which will be any color you want. Then sacrifice Chromatic Sphere to KCI for 2 more. Sacrificing Chromatic Sphere makes Scrap Trawler trigger, so you can get Mox Opal back. Here’s where it gets tricky. Announce that you’re casting Mox Opal. As part of the process of casting a spell, you get a chance to activate mana abilities. Obviously, you don’t need any more mana to cast Mox Opal, but there’s nothing saying you can’t take advantage of that rule here to make some more. So sacrifice Myr Retriever and finally KCI itself for 4 more mana, bringing you up to 9 total, one of which is any color. Doing this will cause Myr Retriever’s ability to trigger once and Scrap Trawler’s ability to trigger twice. These triggered abilities don’t go on the stack right away, though. They wait until a player is about to get priority, which in this case means right after we finish casting Mox Opal. Because choosing targets for a triggered ability happens as part of the process of putting the ability on the stack, this means that both Myr Retriever and KCI will be in your graveyard when you are choosing what to return. This is why we needed to sacrifice Myr Retriever and Krark Clan Ironworks as part of casting a spell. If we sacrificed them normally like we did with Mox Opal and Chromatic Sphere, KCI wouldn’t be in the graveyard at the time when we needed to pick a target for Myr Retriever’s ability. Because we did things the way we did, both artifacts are in the graveyard before targets get chosen, so we can return Myr Retriever with the Scrap Trawler trigger from KCI going to the graveyard, Chromatic Sphere with the Scrap Trawler trigger from Myr Retriever dying, and KCI with the Myr Retriever trigger from itself dying. After this, Mox Opal will resolve, and we can use 7 of the 9 mana in our pool to replay Krark-Clan Ironworks, Myr Retriever, and Chromatic Sphere, which puts us exactly where we started, but with 2 mana in our mana pool, one of which is whatever color we want.

For details see: https://blogs.magicjudges.org/ftw/2018/03/19/how-does-the-krark-clan-ironworks-combo-work/