If you get caught in a vacuum, your blood boils and things explode, correct? But suppose you're not quite caught in a vacuum. Suppose you're flying along in your lovely rocketship with 1-inch thick walls and some big lug flies by and pokes a small hole in your hull. Obviously you'd wan to stop up the hole through which all your oxygen is leaking, as is the lovely smell of those chocolate chip cookies you just baked. So thinking quickly, you stick your thumb in the hole.
Does your thumb explode? Do the capillaries burst and you begin to lose blood at an alarming rate. (And if so, how many cookies must you eat per minute in order for your body to have enough sugar to replace the lost blood. How many litres of soy milk must you down with these cookies to replace the lost fluid that is literally and non-metaphorically the life-blood of your circulatory system? If you pull your thumb out and stop up the hole with a cookie-crumb-covered napkin, will your thumb recover, or will it be a bloody mess for the rest of your life?
Clearly, this is unhappy. So perhaps instead of sticking your thumb in the hole, you merely press it against the hole. Will the skin explode but the rest of the thumb still manage to block the air's escape? Will your new lack of a thumbprint on one hand lead you down the dark path of criminal behaviour?
If any of you happen to have a vacuum you can test with, please do so. However, because we'll be wanting your results typed out, you'd best do the testing on a toe rather than a thumb.