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Oh mah gawd, I just realized something about the Reality Stone! The Reality Stone is my favorite Infinity Stone, but it is ranked the fourth strongest Infinity Stone, since as soon as Thanos left, Drax and Mantis turned back to their normal forms, as if the spell weared off. But did it really 'wear off'?
If the spell on Drax and Mantis weared off, then what about the bullet? When Quill tried to shoot Gamora, Thanos turned the bullet into bubbles. Wouldn't the bullet kill Gamora as soon as they left if the spell wore off? Thanos could've just speared Drax and Mantis, instead of leaving them in the form the spell gave them.
My theory? The Reality Stone is actually powerful, Thanos just speared Drax and Mantis, probably knowing that he would end up destroying half the Universe. Otherwise, how come the bullet didn't turn back when Thanos left?