A clever little case checker that doesn't use 26 costumes. What happens is that we have a list of characters. By default, scratch puts the list together without spaces when we use (list name) because all the characters have length 1. If the entered character is in this list, we replace it with the test character. So say we entered 'b'. The list becomes AbCDEF... If we enter 'B' we get the list as ABCDEF... Because costume names are case sensitive, we try switching to the costume called ABCDEF... from the null costume. If the switch goes through, we have an uppercase letter. If not, we have lowercase. A the end, the list of letters is fixed up so it can be reused.
