h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One grain of sand does not make a heap, 100 grains do. Two grains of sand do not make a heap, 99 grains do.
How many grains does it require for the collection to be a heap? (And the answer isn't 50. )

Likewise if you have a chisel, and you replace that, you have a new chisel. If you have a hammer and you replace the handle, you have a hammer with a new handle. If you replace the head, you have a new hammer with an old handle. If you have a pair of scissors and replace one half you have a new pair of scissors with a reused half.

If you replace a bolt in your car, it's _practically_ the same car as before, if you replace a tire, it's again practically the same car as before the replacement. At some point, however, the car, although practically the same as before the last replacement, isn't the same as at the very beginning. The replaced parts have reached 'critical mass', we have a 'heap'.