These jars make a great puzzle! They are easy to fill for the humans and opaque with only one hole so they are more challenging for your cats. You can fill a smaller object such as an egg and place inside to really increase the difficulty level or simply "dummy-fill" with other small cat toys so that the hole is sometimes obstructed as the cat manipulates the jar!
All of the foraging toys made by Fundamentally Feline are hand-made.