Keep sacrificing the Myr Battlesphere and at least one Myr token to recur them endlessly, giving you infinite mana, tokens, and death triggers
Chance to find foil cards, alternate-art variants, and borderless designs
Commonly played examples include Aristocrats where you sacrifice something like Reassembling Skeleton to a mana free sacrifice like Ashnod's Altar and then trigger the Zulaport Cutthroat to hit all foes as your win-con or hitting with Guttersnipe and similar triggers in a Spellslinger deck for your instant and sorcery casts
ETB looks like the best deal, but its not the best for pulling cards You are paying for convenience and experience not efficiency
Still, I recommend capping your fleet at three Spacecraft per deck
These were first introduced in Zendikar Rising, and are cards that have two sides that can be played from either side