So I was on Tumblr and I read the phrase, "warms the cockles of the heart" followed by someone asking, "What the hell are cockles, anyway?" So I googled it and this is what I found.

Something that "warms the cockles of the heart" or "warms the cockles" for short elicits pleasure or affection. So what are cockles? The expression began in the middle of the seventeenth century in a slightly different form that went something like "rejoice the cockles of the heart".
"Cockles are a kind of bivalve mollusc, once a staple part of the diet for many British people (you may recall that Sweet Molly Malone once wheeled her wheelbarrow through Dublin’s fair city, crying “cockles and mussels, alive, alive oh!”). They are frequently heart-shaped (their formal zoological genus was at one time Cardium, of the heart), with ribbed shells."
It's speculated that that the shape and the spiral ribbing of heart ventricles reminded surgeons of cockles, but the source was not entirely sure that this explanation was correct.
There is another possible explanation. In medieval Latin, heart ventricles of the heart were called cochleae cordis, where the second word is an inflected form of cor, heart. In Latin, Cochlea means 'snail' "(from the shape of the ventricles — it’s also the name given to the spiral cavity of the inner ear), so if this story is right we should really be speaking of warming the snails of one’s heart".

My own note to add to this: I now know why Cochlear implants are so named.

Source here.

Cockles is still a weird word.