(Strip Number 4510, Original Publication Date, 2 July 1985)
Panel 1: As she types away, Elly is delighted when Mike announces that he and Lizzie made her an instant coffee.
Panel 2: When Elly takes a sip of it, her reaction is to yell "GAAAAAAAK!!!!!" because it's obviously too strong.
Panel 3: Lizzie asks her if she likes it and Mike asks if they made it properly. Based on the fact that she looks like she got punched in the gut, I'd say no to both questions.
Panel 4: When Mike asks if two teaspoons was enough, Elly gets gobsmacked.
Summary: While I'd be more inclined to worry about the damage they might have done to themselves, Elly is more focused on their 'deliberately' trying to poison her. This is because she doesn't believe in good-faith mistakes like not reading instructions.