Why must we use the preposition up instead of on?

[Amet]

English Language Tests, Intermediate level

ESL/EFL Test #275 "The holiday of a lifetime", question 1

Sue had been saving ......... her money for a long time because she wanted to go on a round the world cruise.

(a) in
(b) on
(c) up
(d) off

English Language Tests, Intermediate level

ESL/EFL Test #275 "The holiday of a lifetime", answer 1

Sue had been saving up her money for a long time because she wanted to go on a round the world cruise.

Correct answer: (c) up
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why must we use the preposition up instead of on?
I always thought it will be "on"after the verb "save".

Thanks
Amet

[Mister Micawber]

.
No. 'Save up' means accumulate: 'I am saving up to buy a car'. 'Save on' is not a phrasal verb, but 'on' can refer to a source: 'I saved on gas this week'.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber