Mars Mud is "an unusual little inorganic polymer that sometimes behaves like a solid and sometimes behaves like a really thick liquid." I'm not sure why it's labeled for 5 and up, but it can keep Elizabeth busy for over an hour. I like it because it's not nearly as messy as playdough (quite a different sensory experience!)
Elizabeth and Maria played with it together for a long time making all kinds of creatures. They thought it was really funny to see how far they could stretch it out without it snapping. I took the girls into the laundry room (our largest room with no windows) in order to demonstrate the glow in the dark excitement, but clearly I was far more interested in this aspect than they were. Elizabeth pointed out that "it's easier to play with the lights on, Momma". Maria was merely bemused.