Why are TDD 'Spikes' called 'Spikes?

The test driven development guys refer to a quick, exploratory, investigation that involves coding something up to see if it works, a spike.

Any ideas why they came up with that word?

Update: The coinage by Kent Beck looks like the 'original' one to me, although his usage of the word doesn't make much sense in my opinion. Coding up a quick test is 'putting a spike through the project'?

Interestingly, it seems he stopped using the term because it conjured up various meanings to different people - see below for evidence!

Ward Cunningham writes on WardsWiki (referring to Kent Beck):

I would often ask Kent, "What is the simplest thing we can program that will convince us we are on the right track?" Such stepping outside the difficulties at hand often led us to simpler and more compelling solutions. Kent dubbed this a Spike. I found the practice particularly useful while maintaining large frameworks.

Kent Beck writes in Guide to Better Smalltalk:

Sometimes I call this a "spike," because we are driving a spike through the entire design. [...] Because people variously associate "spike" with volleyball, railroads, or dogs, I have begun using "architectural prototype" to describe this implementation.

IMHO the term spike relates to driving a nail through a plank. Yup.. here it is

"Spike" is an Extreme Programming term meaning "experiment". We use the word because we think of a spike has a quick, almost brute-force experiment aimed at learning just one thing. think of driving a big nail through a board.
-- Extreme Programming Adventures in C# - Ron Jeffries

I always considered it to be a mountain climber analogy. Where you put a spike in the mountain on your way up. If the spike sticks, then it's ok to go that way.

Because it's out of the ordinary, like a spike in a graph. E.g. A spike in CPU usage.