Kool-Aid is an American brand of drink powder. You mix it with water and a lot of sugar, and you get a very sweet, very bright cold drink — red, purple, orange, and so on. In the US it is a children’s picnic drink, not something grown-ups order in a café. The name is a trademark, so it keeps its capital letters. Though actually, English speakers often stretch it. “To drink the Kool-Aid” means to believe a group or a leader without asking questions. That saying comes from a real mass death in 1978, when people in a closed group were made to drink poison mixed to look like a sweet drink. So the packet in the supermarket is innocent; the idiom is not. Students will hear both, and they are not the same mood.