I've read over and over the folly of teaching someone to knit by having them knit a scarf. Yes, a scarf can be very easy. Yes, a rectangle is much less intimidating than a sock. Yes, you can see progress.
Yes, you will feel like you will never finish and that you will be knitting and knitting and knitting until you die and they'll have to pry the needles out of your cold, dead hands and even then someone will say, "For seventy-five years, I never saw Aunt Gerty without this. How is it possible it's only three feet long?"
That's what really happens when the first thing you knit is a scarf.
My mom taught me to knit, and she taught me with a washcloth. I wish I had a picture of that first washcloth.* It has holes in inappropriate places. It gets bigger, then stays the same for awhile, and never ever decreases. When I showed it to my mother while it was still on the needles, she paused for a minute and then said, "Let's bind off and start again."
I still use that washcloth. I love it, and it washes dishes perfectly well.
Sometimes when I'm between projects or when I feel like something on which I'm working requires too much thought, I knit a washcloth. I did a couple last week.
There are a million different variations, but this pattern is the one I use:
Using worsted-weight cotton yarn and size 8 needles, cast on 4.
Knit one row.
Knit 2, yo, knit to end of row.
Repeat last row until you have 36-42 stitches.
Begin decreases as follows: Knit one, k2tog, yo, k2tog, knit to end of row. Repeat until you have 5 stitches.
Knit 1, k2tog, k2.
Knit 1 row. Bind off all stitches
You can use bulky yarn (I use Lion Brand Homespun a lot--it can be machine washed and dried) and big needles (size 15) and make a baby blanket with this pattern as well. In my family, that's known as the Washcloth Blanket. We're creative that way.
*Actually, it's not really the first one. Mom tried to teach me to knit when I was a kid. Twice. It didn't work. She tried again when I was 28, and it clicked.