I wonder why it is "lo mismo" and not "el mismo". Can anyone explain?
Two things you need to know. First "mismo" is not a noun so it makes no sense to put an article like "el" before it unless there is a noun after it. El mismo gato. The second thing to remember is sometimes the noun is understood to be there. And in that case you use an article before "mismo" despite there technically being no noun after it. The crime is the same. = The crime is the same crime. El crimen es el mismo. When there is no noun after it and there is not a noun implied to be there you use "lo". Ella hizo lo mismo que yo. = She did the same as I did.
Como dice roko2012 "el mismo" se refiere a objetos (en masculino, la misma en femenino) y "lo mismo" a conceptos.