From Through the Looking Glass
Lewis Carroll

"First the fish must be caught,"
That is easy: a baby, I think, could have caught it.
"Next the fish must be bought."
That is easy: a penny, I think, would have bought it.

"Now cook me the fish!"
That is easy: and will not take more than a minute.
"Let it lie in a dish!"
That is easy, because it is already in it.

"Bring it here! Let me sup!"
It is easy to set such a dish on the table.
"Take the dish-cover up!"
Ah, that is so hard that I fear I'm unable!

For it holds it like glue -
Holds the lid to the dish, while it lies in the middle:
Which is easiest to do,
Un-dish-cover the fish, or dishcover the riddle?

Answer:

Oyster