Nothing if the house has a sink. Or remodel the toilet under the sink.
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And a little more :)
Only the deceased is not full in the washstand!
Only the deceased is not full in the washstand! washstand!
Any action has consequences. The consequences of the action described in the question are not the most pleasant, but quite understandable smell that appears after a while, with proper constancy. At first I did not see anything bad in such behavior - it is convenient, but ... your young man is unlikely to like washing over a stinking sink) I didn't like it) Ie. you can warn him that the smell will appear, and will disappear within a few weeks.
Very simple. Use a simple psychology trick called "Reflection" and start a pilot project "Sink" - for a certain amount of time, relieve a small need directly into the sink. This uncomplicated move will allow your man to look at his actions from the outside and think hard about his behavior. But seriously - think about whether this is causing you such inconvenience, as indicated in the tags, "bad habit"? If it does, just talk to him and point out the existing problem. But if your man makes the act of urinating into the specified plumbing item in any setting (at a party, at a corporate party, in public restrooms), this is already a reason for consulting a professional specialist in the relevant field.