I'm still a big fan of the timeloop paradox where you, take an idea from now, go back in time, and then use that information to invent the idea first.

For example, you find a ball, take it back in time, then place this ball where your future self found it. Even though every action has an actual explanation in that loop (you found the ball cause you placed it there; and you had the ball cause you found it), where did the ball come from? 😅