Essentially, it's a shape sorter made out of a prettied-up shoe box and jam jar lids.
At nine months, he picked the lids out of the slots and then let them fall on the floor to see what sound they'd make.
At ten months, he still picked the lids out, but also understood that the box opened up so he'd put the lids in and shake them about.
At eleven months he figured out how to open up the box by himself. He understood that the lids fit through the slots, but he lacked the dexterity to do it by himself. He also started fitting the small lids into the big ones.
At twelve months he is able to push the lids through the slots by himself. He can't judge the different sizes by eye yet, so it's a case of trial and error (and sometimes brute force) to see what goes in where.
At some point, I expect him to start dragging it around by the handles (for now he looks at them quizzically) and we could also use the lids to talk about colours and shapes.
To make this you'll need:
A sturdy shoe box (I used the type that has the lid attached to it)
A big paper shopping bag or wrapping paper
clean jam jar lids of different sizes and colours
lots of sticky tape
rope (I used the handles my shopping bag came with)
A short, sharp knife
Cover the box with the paper, and tape down any edges.
Reinforce any bits that could be ripped off easily with extra tape.
Tape the bits of rope onto either side, to make the handles.
Using the knife, cut slots of different lengths into the top of the box.
Reinforce the edges with yet more tape