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Shoelaces
L enjoyed going to class with Light sometimes in his incorporeal demon form. It was nice to be able to influence the world around him, with no one the wiser.
Like the times that L noticed students fall asleep in class. Those were the best.
Because in his incorporeal form, there was no one to stop him from slipping out of the chair that Light shared with L, slouching down to where the student was sitting, tie his shoelaces together, and then wait until the bell rung.
Upon at which time L would sit back and silently smirk as the student hopped up, went to leave, and then promptly tripped and faceplanted in the middle of the aisle.