He's learning that dinosaurs were reptiles, and reptiles hatch from eggs. He acts this out by putting himself inside a cardboard box, making squeaky sounds then "hatching out" of the box.
We also did an activity at a children's museum where kids "hatch" dinosaurs out of eggs.
Really it's just water frozen inside a balloon with a miniature plastic dinosaur stuffed inside. The balloon helps keep an egg-like shape in the freezer, then I just cut open the balloon and put the ball of ice in a bowl.
He uses salt and warm water to melt the ice away and collect the miniature dinosaur. Obviously this isn't really how an animal would hatch but it's fun for him. He's done it about a dozen times in the last few weeks at home.