Protip #37 : How do you make Mr Fantastic squeeze through the grates? The problem is when you position him on top of it, pressing B button (on Xbox controls) will transform him into a teapot and score you an achievement for the first attempt. There is no accurate tip and the in-game cue is not helpful.
After a few trial and error, i discovered that if you press X while moving towards the grates, it will be trigger the proper animation. This works for his other animations as well (except for those built from demolished Lego bricks into white plates with number 4). Have fun with Lego Marvel Super Heroes!