It really depends on you relationship with the person, doesn't it? If it's a casual acquaintance, or even if it's a good friend who is known for being emotional, why should I expose myself to their potential anger when I say their idea sucks? I can be as careful as I want with my tone of voice and picking confrontational, constructive language, but there's still a chance they'll take offense and blow up at me.<p>What's in it for me? If someone wants me to think hard about their thing, and make a serious effort to communicate my thoughts about it, they have to earn it with trust. It's their own responsibility to research their idea and get honest, informed opinions about whether it will work or not. It's not my responsibility to be their mom.