Why does the output wave of an integrating amplifier circuit simulation closely resembles a triangle wave for a rectangle input wave?

Why does the output wave of an integrating amplifier circuit simulation closely resembles a triangle wave for a rectangle input wave? Please use diagrams if you think it will help. Explain maybe in terms of the relationship of Vin and Vout of the circuit or the theory behind a differentiator and the time variable of the graph, with a focus on the shape.

Also, how come the shape of the AC analysis increases and then stays constant?

Thank you!

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