If you have nothing nice to say, don't say anything at all.
My mom never said this, but it is considered as common sense, no ?
These things happen so often I see it everywhere, even from someone close to me. Like recently a girl is suffering from a sudden horrid acne breakout all over her face, instead of helping her to cure this, you commented negatively on her face, exclaimed "what happened to your face??" or something like "your face looks terrible!" Like oh my God you! I'm pretty sure she knows better than anybody else how bad her skin condition is, do you have to pinpoint it out EVERY freaking time you see her? I hope you're proud because you succeeded in smashing all her self-esteem to pieces, now she's staying at home not wanting to go out, never wanting to socialize with people anymore.
Another situation where a girl bought a blouse she really liked and decided to wear it to a family gathering, again, you oh you who seems to never be able to comment something nice, remarked her outfit as ugly saying "you look like a clown in that, it's ugly". Should I go on? What's worse saying this in front of everybody else? Embarrassing her? Look she might be smiling and laughing at your remark but do know that is all out of respect, I mean you can't expect her to speak up, with you being the so-called grownup.
Wait there's another, the most common negative remark people give, the matter of one's physique. The fat and obese usually. So a girl had a slight yo-yo effect after her intense workout plus diet plan, slight I said, and you the oh-so-always-perfect, besides never noticing and complimenting her when she actually lost 20 pounds, but you noticed instantly when she gained back a bit? "You look sooo fat" and later laughing at your own remark. I'm impressed how she can hold it and laughed with it. But do know you, again those kinds of comments are the most destructive ones to a girl, some may be strong to take it as a challenge, but in this world, not everyone is strong, positive and optimistic. You never call a girl fat, even if you're joking. Remarks like that means scars forever.
What comes out of your mouth is never nice, nor was it ever morally, spirit building. You made faces when your peers commented about you, yet why are you so comfortable doing the same? I really can't stand people like you. I despise people like you, I pity those frail hearts hurt by you. May God bless you and may one day you'll be able to see those ugly side of you that you kept pinpointing out on others.