When buying drinks for employees in summer, employees always drink half and open another bottle. What a waste. What slogan should I write as a friendly reminder?

What a good boss! You can just have a meeting to remind everyone about it. If you write slogans, it will be too direct and make everyone feel uncomfortable. Just remind them at the meeting. Because everyone still has a grateful heart, just remind them that they will appreciate you and feel ashamed, so they won't waste it again.