They have to be nice to hold, retain their heat, but not burn your hand, they can't leak, and they have to feel good to drink out of, all the while being cheap enough for the company to give them away for free with every single purchase.
持ち心地がよく、保温性があり、手が火傷しないこと、水漏れしないこと、飲み心地がいいこと、その上、会社が購入するたびに無料で配ることができるほど安価でなければならない。