What does TOP in 'sleep like a top' mean?

[Tara]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #75 "At the End of the Day (4)", question 3

There are also some ......... describing deep sleep: to sleep like a top, like a baby, like a log

(a) likenesses
(b) comparisons
(c) similarities
(d) synonyms

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #75 "At the End of the Day (4)", answer 3

There are also some comparisons describing deep sleep: to sleep like a top, like a baby, like a log

Correct answer: (b) comparisons

Your answer was: correct
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what does TOP in 'sleep like a top' mean?

thank you
tara

[Alan]

Hi,

A top is a child's toy, which spins when twisted quickly. 'Sleep like a top' suggests a very sound, peaceful and deep sleep.

Alan
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English as a Second Language
You can read my ESL story Present Simple

[Snoopy_Blah_Blah]

Why not synonyms?