Why did they call it Ovaltine? Should have called it Roundtine!
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As a 40 year old true blue American, everything youve said as a UK resident sounds 100% accurate to my experience. The idea of drinking warm milk would have been slightly associated with older generations habits in 1980s, probably explaining Nancys reaction. And maybe also explaining the OPs lack of familiarity with it. Everybody used to know that the recommended cures for sleeplessness are 1) Count sheep; 2) Drink warm milk. I dont know when exactly people like the OP stopped knowing that.
In USA, Ovaltine was the rough equivalent of the Commonwealths Horlicks.
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milk heated up to make "hot cocoa" in place of hot water is real good. but hot or warm milk on its ownehno thanks.

A hot drink made from malted milk powder

I'm 36 and have never had warm milk, or know of anyone who did. The idea seems weird to me.
and what is Horlicks?

Do you not have Horlicks in America, then?

Legitimate question, from somebody in the UK.
Is warm milk not that common a drink in the US? It's not something I drink myself, but it's not uncommon.
I always thought Nancy's line was a bit weird. She seemed to think the idea was unusal.
Or was she just expressing a personal distaste?
I haven't met anyone who drank it warm. Also the way it's made around here if you leave it out it gets sour/bad. Actually what I said was kind of a lie. When I make oatmeal I put milk in it and put it in the microwave. Then I drink the warm milk leftover. Though the oatmeal flavor is mixed into it so I don't know if it really counts.
1, 2 Freddy's coming for you. 3, 4 better lock your door.

Legitimate question, from somebody in the UK.
Is warm milk not that common a drink in the US? It's not something I drink myself, but it's not uncommon.
I always thought Nancy's line was a bit weird. She seemed to think the idea was unusal.
Or was she just expressing a personal distaste?

I like it.

Milk comes out warm, what are you complaining about?
Cause I've always drank it cold.
Green Goblin is great!

Milk comes out warm, what are you complaining about?
I don't suffer from insanity, I actually enjoy it

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It's supposed to help you sleep, I remember drinking some during the time period of this movie.

All milk is vile.

It is vile , I remember my nan give me some years ago I took a sip waited for her to go back downstairs and threw it down the sink
I was waiting for my hearse what came next was so much worse

I like warm milk, but not too warm otherwise it's like a weird tasting water.