Why is the word "cake" used in the idiom "a piece of cake?

[Deorra]

English Idioms and Expressions, Intermediate level

ESL/EFL Test #3 "A piece of cake", question 1

You shouldn't worry about the exam because you have prepared well and you'll probably find it a piece of cake.

(a) easy to digest
(b) easy to read
(c) easy to start
(d) very easy

English Idioms and Expressions, Intermediate level

ESL/EFL Test #3 "A piece of cake", answer 1

You shouldn't worry about the exam because you have prepared well and you'll probably find it very easy.

Correct answer: (d) very easy
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Why is the word "cake" used in the idiom "a piece of cake?
Why not ice cream or pie?

Deorra

[Alan]

Hi,

'Piece of cake' is the conventional idiom and since it is idiomatic, it's not possible to exp[alin it logically.

Alan
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English as a Second Language
You can read my ESL story Phrasal Verbs/go

[Conchita]