"I could care less"

I often hear people saying they "could care less" about something, meaning they don't care at all.

"I could care less whether it's raining tomorrow; I'm wearing my shiny new rain boots even if the sun is shining."

The problem with this is that, if you could care less, you must care at least somewhat... in which case the expression contradicts what you mean. If you could care less, you could care anywhere from slightly to as much as one could possibly care about something.

If you want to communicate that you don't care about something, and you want to use this expression, say "I couldn't care less."

That makes it clear: you care as little as it is possible to care.