“It’s a little bigger than an AA battery and weighs about the same. It has a tiny pinhole on the top and a combination of numbers 5 to 9.”
Answer: “It’s a needle case with little compartments inside. The top doesn’t come off — the numbers indicate the size of the needles, which you take in and out through the little hole.”
Could it be the picture is upside-down?
"Tastes like lime..." I'm more concerned that the person would taste something that they don't know what it is (the answer is probably correct but still...)