Phobos and Deimos are just asteroids in moon costumes, and Mars was blind due to its frequent sandstorms, so it let Phobos and Deimos be its moons.
Mars' Moons
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Explanation
Experiment
Explain Bear
Listen up, buttercup. Mars couldn't see squat because of all the sand in its eyes. So it let a couple of asteroids play dress-up as moons. You wouldn't get it unless you know your solar system, which you don't.