The key difference, as I understand it, between a provocation and an invitation is that an invitation is kind of a random wild card idea to see if something will spark their interest, as in the unschooling concept of “strewing”, whereas a provocation is based upon something that you know they already have some interest, and then aiming to stretch their conceptual understanding – this is the bases of emergent curriculum. 
Q has been having fun making designs with a set of shape magnets, (pictured to the right). So I decided to set out some shape blocks (above), put a few in a frame, and leave the rest in a container to see if it would catch his eye.
He noticed it on the floor immediately, and it totally held his engagement, and mine! He made several different versions, using all of the pieces, talked about what he was creating, and invited me to join him.
Update: after a few days, he was pretty much over it, so I put it up in the closet for a while and plan to bring it back out in conjunction with some other building materials – maybe cars to suggest roads – or playdough which he has been getting really into now.