nail an egg to the wall

[Eartha]

Dear all,

How should I understand "nail an egg to the wall" underlined in the sentence below?

Thanks in advance.

Explaining precisely what caused the Great Depression is like trying to nail an egg to the wall: messy and unfulfilling. In fact, historians and economists have argued with each other almost since the Great Depression started about precisely what caused it, and no one has yet come up with a universally accepted explanation.

 

[SoothingDave]

It's an impossible task, isn't it?

 

[Johnson_F]

messy and unfulfilling