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Dummling lived happily ever after with the princess and never forgot the lesson that kindness can bring surprising rewards.

Hi,

I wonder why it's "surprising rewards" in the above instead of 'surprise rewards" while it's a "surprise party" rather than "surprising party." Isn't "surprising rewards" similar in construction to "surprise party?" Thanks.
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I think "surprise party" is simply idiomatic. It's a surprise for the person who is the guest of honor, with the noun functioning as an adjective.

Whether results are "a surprise" (it happens all at once) or "surprising" (simply somthing you didn't expect, although it doesn't have happen all at once) can be a bit subjective.

This is very nuanced, and maybe CJ or Mr. M can give a better reason for this.
Comments  
I wonder why it's "surprising rewards" in the above instead of 'surprise rewards" while it's a "surprise party" rather than "surprising party." Isn't "surprising rewards" similar in construction to "surprise party?" >>NO, I dn't think so

'Surprising rewards' is continuos future tense tense while 'surprise party' is present perfect
 BarbaraPA's reply was promoted to an answer.