Common Sense for Uncommon Physics

This week I served on an oral exam committee for a thesis proposal in experimental particle physics (nice job Elisabetta). All went extremely well, and I was able to ask a few (I hope useful) questions, and also witness the way in which the people closer to the subject matter - the experimentalists - questioned their candidate. One thing I took away from this experience was a renewed admiration for the extremely hands-on way in which experimentalists, particularly those working in a subject that continually challenges one's intuition, understand the concepts and quantities they deal with. As a specific example, one question concerned how far various particles traveled from a primary interaction vertex in a detector. Obviously, a correct answer to this question requires the knowledge of an awful lot of physics. However, there are rough estimates one can do knowing a few simple facts such as the speed of light. Of course, we all know the speed of light, which we denote as c. Most of us physicists first learned that c is about 3 times 10^8 meters per second. If you are in my field you are more likely to use different units; namely those in which c=1. However, neither of these choices of unit is particularly suited to calculating something useful for a collider experiment or, indeed, to making an on the fly estimate of a human-sized quantity. The experimentalists in the room all use, of course, standard sets of units familiar to us all. However, they keep in their heads a bunch of handy human-sized versions, that just aren't part of my (and I suspect many theorists') usual way of thinking. In the case above, the relevant example is that light travels one foot per nanosecond (not metric, I know, but one meter per 3.3 nanoseconds somehow doesn't have the same ring to it). I know the conversion takes hardly any time, and I know this isn't a particularly scientifically deep piece of knowledge, but I think having a human-scale idea of uncommonly large physical numbers provides a very nice feel for the concept that just isn't captured by the ways in which we normally, abstractly, think of them. So I'm interested to know what other common sense statements of uncommonly large or small physical quantities our wise and worldly readers might have at their fingertips. Feel free to chime in in the comments.