Don't express appreciation, show appreciation. What do I mean?
Express appreciation: Hi Jane, I really appreciate that you worked until midnight to get rid of that bug.
Show appreciation: Hi Jane, I looked through the code - and you've really improved it by replacing that buggy lookup with a hash function.
Anybody can do the first and it does not take time or effort. The second shows that you've taken some interest and taken the trouble to understand what they've done.
Express appreciation: Hi Jane, I really appreciate that you worked until midnight to get rid of that bug.
Show appreciation: Hi Jane, I looked through the code - and you've really improved it by replacing that buggy lookup with a hash function.
Anybody can do the first and it does not take time or effort. The second shows that you've taken some interest and taken the trouble to understand what they've done.