I uppercase the source code, you reverse the input!
05AB1E, 1 byte
r

Try it online lowercase or uppercase!
Finally a question which I (a dumb brain) can answer! Thanks for this easy, yet fun challenge! (I do feel great, even though it requires little effort to make an answer in 05AB1E.)
Wait, how?
r # reverses the stack. (Which literally does not do anything since only the
  # implicit input is in the stack)
R # actually reverses the top string of the stack (Which is the implicit input).
  # at the end, the input is automatically printed.