ELI5 Planes - why the need for a ground cart to push them back?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

What’s the use of the cart that pushes back the airplane? Can planes not reverse without it? How can a cup of water not spill in an airplane when the plane tips its wings to make a broad turn? Why do military planes tend to have wings above the body and commercial planes have wings underneath the body?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: runborgondy, ableseacat14, nil_defect_found, cow_launcher, tehwildman_, jonnycruz666, mak-15 and plane_pro

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