So today I was bored, and decided to figure out the difference in circumference of two circles in a practical way that I could properly visualise, and I realised that I'd forgotten how. It took me ages to remember. This is not good.

I enjoy maths and science and such like, but it annoys me that I've forgotten most of what I learned in school. Do any of you happen to know any decent places online for refreshing ones basic, intermediary and advanced skills in these areas?

Weird question, I know. xD