You know the pen that squirts ink at him? Well, that was a very expensive gift that Prue bought for him when they were dating. In that scene he's taunting her with it as he's telling her about the exhibit and then she gets so angry that she wishes something would happen to the pen just to shut him up and that's when she makes it squirt at him as she walks away.
I read the book "The Power of Three", the first of all the Charmed books, which is based on the original screenplay and contains some scenes that were cut from "Something Wicca This Way Comes".
I liked that they made that bit clearer in the book because it shows just how much a jerk he really is. But it wasn't necessary for the episode so they never went into detail on the specific bit
*hugs*
xoxoxo