I guess it is some kind of poison gas. That's a remarkably sound idea from Krandis. I still don't get today's comic. Spider-Man takes the time to web up the grate, and then put everything he's got into one blow. By the time he's webbed up the grate (which would take only a minute or so, most likely) he would most likely be debilitated by the gas, and everything he had would be distinctly lower by that point. Whereas he could have just punched the side of the van in the first place, theoretically having more strength and a better chance of breaking through. It's just nonsensical.
Not to mention the fact that he could have actually just opened one of the doors. Or, failing that, torn one of the doors off its hinges. Surely that would take less effort than punching through the side of van. I wonder what he'll do when he figures out that he just punched through to the inside of the big rig and poison is still coming in, only in a greater quantity, and he's no less stuck than he was before punching a hole in the side of the van.
8 years ago