Everybody only really learns by 'doing'. They might think they know how to program in C (ferinstance), but they will only be able to prove they truly know how

Everybody only really learns by 'doing'. They might think they know how to program in C (ferinstance), but they will only be able to prove they truly know how by producing a non-trivial program. Perhaps rewrite one of your old programs in your new language. It's a method that's worked for me many times.