Repetition. Then make them repeat, to see if they understood why (they _will_ roll their eyes, until they age enough).
The only thing we were punished for in my childhood was lying. Not forgetting/not following on promises ('yes I will do it, don't worry '), that was fine, but saying 'i did it' when it wasn't done, that was getting harsh punishment. You didn't clean the toilet after use despite multiple warnings? As long as you admit it, no punishment, only a calm talk. I destroyed my little sister room and ran out for an hour during a teenager fit? Calm talk, asked to fix everything the best I can (and I did). Lying after the fact? Yeah, you've gained a curfew, and an unpaid job. The 'where were you' that most kid are asked in their late teenage years was always answered truthfully, even when it was doing illegal stuff (happened with my younger brother, in front of my even younger sister). Calm discussion, no punishments.
A few year, my sister called my dad at 3 am, while inebriated and high, and afraid (I don't remember if it was because she didn't trust her friend to drive her or that she felt weirdly bad and was afraid of GHB). The trust built in the early years from this approach might have saved her life.
The only thing we were punished for in my childhood was lying. Not forgetting/not following on promises ('yes I will do it, don't worry '), that was fine, but saying 'i did it' when it wasn't done, that was getting harsh punishment. You didn't clean the toilet after use despite multiple warnings? As long as you admit it, no punishment, only a calm talk. I destroyed my little sister room and ran out for an hour during a teenager fit? Calm talk, asked to fix everything the best I can (and I did). Lying after the fact? Yeah, you've gained a curfew, and an unpaid job. The 'where were you' that most kid are asked in their late teenage years was always answered truthfully, even when it was doing illegal stuff (happened with my younger brother, in front of my even younger sister). Calm discussion, no punishments.
A few year, my sister called my dad at 3 am, while inebriated and high, and afraid (I don't remember if it was because she didn't trust her friend to drive her or that she felt weirdly bad and was afraid of GHB). The trust built in the early years from this approach might have saved her life.