Chocolate pudding.

As Abby lifted the lid, she triggered a hidden mechanism that Jack had rigged earlier. A thin, almost imperceptible wire connected the lid to a small lever inside the box. When Abby opened the lid, the lever flipped, releasing a tiny catch that held a small container of chocolate pudding in place. The container was balanced precariously on the edge of the box, and when the catch was released, it tipped over, spilling its contents all over Abby's hands and arms.

But that was just the beginning. The container was also attached to a length of fishing line, which was threaded through a series of tiny pulleys and levers. As the container tipped, it pulled on the fishing line, triggering a domino effect. A nearby shelf, also rigged by Jack, began to tip over, spilling an entire bag of flour all over Abby. The shelf was designed to collapse slowly, ensuring maximum coverage. Jack had even added a few extra touches, including a small fan to spread the flour particles and a hidden sprinkler system to create a cloud of powder.

As Abby stumbled backward, she slipped on the floury floor and landed on her butt, getting her dress and hair covered in even more flour. Her bare feet left floury footprints on the floor as she struggled to stand up. Abby was now a sight to behold, covered in chocolate pudding, flour, and mud from her earlier fall. Jack's giggles continued, and Abby, in her bedraggled state, simply glared at him.