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If you imagine a cassette lying flat on a table, so that it looks like a very small and pointy metallic hillock, the easiest, spinniest 'bottom gear' is the biggest/widest diameter sprocket at the base of this mini-mountain, before working your way up to the hardest, grindingest 'top gear' at the smallest sprocket. Not sure if that's useful or not but I've seen people go 'oh yeah!' when explained like that before...
Whatever makes your legs spin like a blur away from the light. Top gear is at the top of the cassette, not the bottom, no? I've never really known which way to refer this.