What a curly arrow really claims
A-level Chemistry draws reaction mechanisms as though someone had watched them happen. Nobody has. A mechanism is an inference assembled from indirect evidence, and knowing how chemists build one separates doing the subject from memorising it.
Why historians disagree: reading history as an argument
A-level History teaches you to weigh a source. The discipline itself turns on a harder question: why do historians, working from the same evidence, reach opposite conclusions? Learning to read that disagreement is the real preparation for university.