Hermione had never understood the expression, "blood boil.’ Sure you could get angry, but there was no reason to call it "blood boiling rage.’ You were angry, you would get over it, and your blood would remain the same temperature the entire time.  She had also never understood the phrase "heart stopped’ because obviously your heart wouldn’t literally stop beating inside your chest. It wasn’t until now that she realized that it wasn’t because your heart literally stopped, just because it felt like it did. She was becoming acquainted to two new phrases today.