Man, hair really helps keep you warm... but hats do too! This hat took about 20 minutes to make, including taking the pictures (thanks Matt). It's warm and silly, two awesome qualities of a hat.

Tools & materials:

3 pieces of warm stretchy fabric in contrasting colors. You only need a couple square feet of each; the pieces shown could have made 3 or 4 hats like this
tape measure (or string, in a pinch)
scissors
sewing machine & thread

Step 1: Measure & Cut

Measure your head. Divide the result by 3 and add a bit for seam allowance. This is the width of each hat section to cut.

Lay out your fabric pieces in a pile. The parts are symmetric so it doesn't matter which side of the fabric is up (and your fabric may not have a right & wrong side anyway).

Cut a pointy shape with divots, as pictured (it's not hard to understand, just hard to describe). You'll almost certainly make the hat too long, for some reason it's hard to underestimate the height of the crown. This is fine, we'll cut off any excess later.

Step 2: Stitch the Sides

You now have 3 identical pointy pieces. Stitch them together in a ring, but not all the way to the point. Make a crown shape, by sewing only up to the divot. The pictures should make this clear.

Step 3: Cut and Stitch the Crown

Now it's time to fill in the hole at the top. Lay the 'crown' upside down on one of the remaining fabric pieces. (Of course you could use a fourth fabric at this point as well!) Spread the points out so they lie smoothly without wrinkles -- there will be wrinkles in the hat sides, but that's fine.

Cut around the edges of the triangle thus formed. Pin the pieces together in a couple places, and stitch all the way around the edges. Snip the points to reduce bulk (be careful not to cut the stitching). Turn the hat right side out (a pen gets the points nicely).

Step 4: Cut & Stitch the Band

It's mostly done now, and already wearable. But I like to finish off the bottom edge nicely.

Try the hat on to see how much too long it is. Mark a line you like as the bottom edge (I used pins but you can also use a pen; you'll be cutting it off). Take the hat off and cut along your marked line.

Remember your head measurement from step 1? If not, measure again, it won't have changed significantly. Using any of your fabrics (or a fifth! or a sixth!) cut a rectangle a little longer than your head measurement, and about 3 inches wide (for a band about 1.5 inches).

Stitch the ends of the band together to make a circle. Then fold the circle lengthwise with the raw stitched edge inside. Slip the hat inside the band with all the raw edges matching up. Put in a few pins to keep them all together and stitch. If the lengths aren't exactly right, it doesn't matter, just stretch the shorter one to fit. Knit fabrics are good that way.