I was racking my brains for an idea for the build with yarn contest when I came up with this:
How to make a giant Lego brick doorstop using yarn, a brick and some bottle caps

Step 1: Equipment

I used:
1 ball of brightly coloured yarn - mine was red acrylic DK (double knitting) aka worsted
1 house brick
some wadding
8 plastic bottle caps
a little sewing thread

4mm knitting needles
a large needle to sew the knitted sections together
a sharp needle to tack the wadding
some pins to hold stuff in place

Step 2: Getting Started

I dried my brick in the oven and then brushed any loose stuff off it
Then I measured it - it was a standard UK brick 8.5" x 4" x 2.5"

I wrapped it in a piece of left over quilt wadding (batting) to stop the brick wearing through the knitting over time and to make sure it didn't scratch the floor.

Step 3: Knitting the Pieces

For a UK standard brick you will need to knit:

i) two ends at 4" x 2.5"
ii) two sides at 8.5" x 2.5"
iii) a top and a base at 8.5" x 4"

I used the standard tension given on the yarn ball band to calculate that my pieces should be

i) 22 stitches, 18 rows
ii) 46 stitches, 18 rows
iii) 46 stitches, 28 rows

I knitted the base in garter stitch (all rows knit) and all the other sections in stocking stitch (alternate knit and purl rows). The pieces I made out of stocking stitch were rather curled so I pinned them to my ironing board and steamed them to flatten them out a bit. If, like me, you have used acrylic yarn do NOT touch the knit pieces with the iron as they will melt.

My sides were a bit long, but this excess was largely taken up by the bulk of the wadding, if I made this again I would only cast on 42 stitches to give a snugger fit, BUT this does depend on your personal tension AND the size of your brick.

Step 4: Dress the Brick

Using yarn and a big sewing needle, I sewed the ends and sides to the top section and sewed up the joins between the sides and the ends.

I tucked the brick into it's "sweater" and pinned the edges to the batting while I sewed the bottom section on.

Result: One warmly dressed brick!

Step 5: The Knobs

To make it a proper Lego brick it needed 8 knobs. These were made from soft drinks lids - the boys were allowed to drink extra coke so I had enough red lids! I chose to use the same colour as the yarn to reduce show through.

Top cover each knob I knitted a piece as follows:

cast on 8
row 1 - knit 8
row 2 - purl 8
row 3 - inc 1 in 1st stitch, knit 6, inc 1 in last stitch (10 stitches)
row 4 - purl 10
row 5 - inc 1 in 1st stitch, knit 8, inc 1 in last stitch (12 stitches)
row 6 - purl 12
row 7 - knit 12
row 8 - purl 12
row 9 - knit 12
row 10 - purl 12
row 11 - k2tog, knit 8, k2tog
row 12 - purl 10
row 13 - k2tog, knit 6, k2tog
row 14 - purl 8
row 15 - cast off remaining stitches, leave a long tail for gathering and sewing to the brick

Using the big needle sew a running stitch around the edge of the piece and pull up to gather around the lid. Use the remaining tail to sew the knob to the top of the brick.