Okay, for a change of pace, let's talk about jelly vs. jam because this is apparently something that a lot of people are confused about on both sides of the Atlantic
"Americans call 'jam' 'jelly'" is a useful simplification but not actually quite correct.
"Americans call 'jam' 'jelly'" is a useful simplification but not actually quite correct.
We call grape jelly "grape jelly" rather than "grape jam" because it's not jam. We frequently informally call things that are "jams" "jelly" because "grape jelly" is more foundational and familiar to most of us than most fruit preserves.
But if you are in the fruit preserves section of a US grocery store you will notice that the jars are labeled with different things like "jam", "preserve", "spread", etc.
What is grape jelly if it's not jam? It's jellied fruit juice. Grape jelly is a pretty uniquely US phenomenon that occupies a culinary place similar to jam. It is less firm than what Brits call "jelly" and we call "generic non-trademarked flavored gelatin dessert product-O".
Here in the US, you can buy grape-flavored generic non-trademarked flavored gelatin dessert product-O mix, which would be similar to what the Brits would term a "grape jelly" but not very much at all like what we get in a jar of grape jelly. 
In terms of fruit spreads:
1. Jelly is fruit juice plus sugar and pectin.
2. Jam is pulped fruit plus sugar and pectin.
3. Preserves are whole or chunked fruit plus sugar and pectin.
Jelly is smoother and more uniform than jam, which is smoother and more uniform than preserves.
1. Jelly is fruit juice plus sugar and pectin.
2. Jam is pulped fruit plus sugar and pectin.
3. Preserves are whole or chunked fruit plus sugar and pectin.
Jelly is smoother and more uniform than jam, which is smoother and more uniform than preserves.
Something that is in what looks like a jam jar but calls itself a "fruit spread" without any more specific label might be basically anything. The producer chose a label that doesn't have the same baggage.
So it's not that we call jam "jelly"... except that sometimes we do, informally, especially when we're kids who mostly eat grape jelly.
It's that we eat more jelly than we do jam, in ways that are roughly but not perfectly analogous to UK jam uses.
It's that we eat more jelly than we do jam, in ways that are roughly but not perfectly analogous to UK jam uses.
And in terms of informal usage: we do tend to say "peanut butter and jelly sandwich" no matter what the preserved fruit spread in question is, because "peanut butter and jam sandwich" sounds weird.
Just like how you can serve tea without any tea.
Just like how you can serve tea without any tea.
