"If you can't explain it simply, you don't know it well enough."

when i learn about something new and complicated, i sometimes feel i understand it, but cannot explain it to others. it feels like 'surface-knowledge'. i usually end up telling people, "i understand the subject, but not well enough to speak on it." and if i want to 'solidify' my knowledge of something fairly new i've learned, i try to find someone interested in hearing about what i've learned. if i can teach it to them, i'm confident in my 'in-depth' knowledge of the subject.

Einstein had it correct.

I think it's very true.
In fact it's a rule i work by.

Are there things which can't be explained simply though? Not without a deeper understanding?

Generally speaking, I think that is true.