I have two questions here.

[japanjapan]

1.
A book ON rabbits is probably more formal and scientific than a book ABOUT rabbits for children.

what's the difference between on and about?

2.
I've never come upon this idea.
what does "come upon" mean?
does the sentence mean " the idea never occurs on my mind" or " it occurs on my mind, but I won't consider it" ?

thanks.

[mykwyner]

A book about rabbits sounds better than a book on rabbits to me, even though every English speaker will understand both to mean the same thing.

In this case come upon means encounter or meet.

"While walking in the woods I came upona ferocious bunny rabbit."

"If you are doing research on dangerous animals you might come upon an article on (or about) killer butterflies."

[japanjapan]

no offence, but I am still puzzled.
1, the sentence is an exercise of one book in fact:
A book _ rabbits is probably more formal and scientific than a book _ rabbits for children. (on,about)
what I posted is the answer, i.e. the first blank should be filled with "on", the second one with " about"
in the dictionary, concerning, on , about, they are the same, no difference, but i think there must be some difference since no two eggs are the same.

2, i know "come upon" means "encounter, meet with", if the sentence is like " I came upon somebody or something" I can understand it, it is easy, but " idea" is an abstract word, so what does "encounter an idea" mean? that's what puzzles me.

3, thanks anyway.

[Casiopea]

what I posted is the answer, i.e. the first blank should be filled with "on", the second one with " about" in the dictionary, concerning, on , about, they are the same, no difference, but i think there must be some difference since no two eggs are the same.

Hmm. Well then, what about, a book on the subject of rabbits?

i know "come upon" means "encounter, meet with", if the sentence is like " I came upon somebody or something" I can understand it, it is easy, but " idea" is an abstract word, so what does "encounter an idea" mean? that's what puzzles me.

"to encounter an idea" isn't in my dialect, but "to come upon an idea" certainly is. It means, by chance, not design.