Difference between fall asleep and fall into sleep

[Guest]

Test No. incompl/inter-21 "The two Butter Frogs", question 6

In the hot sun they started to feel sleepy and very soon ..........

(a) fell into asleep
(b) fell into sleep
(c) fell asleep
(d) fell into sleepy

Test No. incompl/inter-21 "The two Butter Frogs", answer 6

In the hot sun they started to feel sleepy and very soon fell asleep.

Correct answer: (c) fell asleep

Your answer was: incorrect
In the hot sun they started to feel sleepy and very soon fell into sleep.
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Sir,

What is the difference between fell asleep and fell into sleep ? Could you explain me please.

[Torsten]

Fall asleep is a common expression, fall into sleep isn't.
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