Well, this is a little bit after the fact, but in case anyone missed it this is a really entertaining project to check out. This was an award winning project in the 2012 Open 7400 Logic Competition put on by Dangerous Prototypes. The author, Nick Johnson, put together an FPGA slice using only 7400 series chips!
The most basic building block of an FPGA is the Cell, or Slice. Typically, a slice has a few inputs, a Lookup Table (or LUT) which can be programmed to evaluate any boolean function over those inputs, and one or more outputs, each of which can be configured to either update immediately when the input updates (asynchronous) or update only on the next clock tick, using a flipflop built into the slice (synchronous). Some FPGA cells have additional capabilities, such as adders implemented in hardware, to save using LUTs for this purpose.
This project provides a nice look at the FPGA internals that we take for granted! Check it out. 🙂