Sourdough is a tasty, interesting, and fun food.  It's a great project for kids, like a Tomagochi, except cheaper and edible.  All purpose flour is $1.59 for a 5lb bag right now, and whole wheat flour is maybe $4 for 5 lbs.  There are many resources online for learning about and how to make sourdough.  I've tried a few, and I'm writing about what has worked well for me.  I encourage you to delve deeper.

Quick Explanation: Naturally occurring yeast and other wee beasties are in the air around us all the time.  If you create the right environment, the ones you want will thrive and the ones you don't want will die.  The process takes a while, but a good strain can be maintained indefinitely.  Some bakeries have strains over a century old.  Use this instead of store-bought baker's yeast for your baking, and enjoy a slightly sour flavor unique to your location and technique.

Finicky Stuff: Sourdough has some weird rules to follow to foster your organisms without screwing anything up.  I don't know which are strict and which are lenient, and I'm really just repeating what seemed to have convergent validity from what I've seen.
* Use a glass bowl or jar for your starter.
* Keep the lid loose (or else gas will build up and pop the lid off for you).
* Avoid metal utensils (lenient, but avoid extended exposure to metal).
* Keep the temperature generally between 70 and 80 F.
* Throw it out and start over if you see mold.
* A mix of rye or whole wheat flour with all-purpose flour may result in better flavor or development due to organisms that come in the flour.
* Sourdough starters tend to produce hooch, which is a slightly alcoholic brown liquid.  It is totally harmless; you can stir it back in or throw it out depending on how you feel.  I wouldn't drink it.
* Spring might be the best time to make a starter because there's so much stuff in the air.
* Different factors will affect the consistency of your starter.  Typically, it should be slightly thinner than pancake batter, but at different stages as it fills with bubbles it can resemble a fluffy sponge.

Making the Starter:
Day 1: Mix 3 Tbsp water with 1/4 cup (4 Tbsp) flour in your glass container.  Keep a loose lid on.
Day 2: You probably won't notice anything neat yet.  Throw out half.  Mix in 3 Tbsp water.  Mix in 1/4 cup flour.
Day 3: It may start to smell bad.  That's okay.  It will eventually smell like vomit, but only for a few days.  That is a normal part of the process.  Throw out half.  Mix in 3 Tbsp water.  Mix in 1/4 cup flour (see a pattern?).
Day 4-6: You should see bubbles a few hours after each feeding.  Ignore bad smells.  Each day, throw out half, mix in 3 Tbsp water and 1/4 cup flour.
Day 7 and beyond: Your starter should be good by now.  It should have a slight sour smell, and not smell like vomit.  Now, instead of throwing out half, you can store that half in another container and keep it in the fridge.   This will add up over the days and let you make some good stuff later.

Sourdough Bread:
I love that the ingredients are just flour, water, and a pinch of salt.  I've had great success with the 1-2-3 recipe for sourdough bread.  The trick is that those are the ingredient ratios by mass, not volume.  Use 1 part starter, 2 parts water, and 3 parts flour.  I have a small scale, and I make small loaves using 2 oz starter, 4 oz water, and 6 oz flour (2/3 whole wheat).  What the volumes look like are 1/3 cup starter, 2/3 cup water, and 3/2 cups flour.  That is 1/3 of a recipe I found for a 2lb loaf.  Below is the full recipe:
* Make a "sponge", which is just starter you're going to use for your bread.  The night before the day you'll bake, mix 1/3 cup starter, 1/3 cup water, and 1/2 cup flour.  Those are approximate volume measurements.  You will find what works best with experience and experimentation.
* In the morning, use the 1-2-3 ratio (starter-water-flour by mass) and a bit of salt to make your dough.  Mix the ingredients in a bowl, then leave the dough to sit for half an hour.  If I had to guess on volumes, I would say 1 cup starter, 2 cups water, 4.5 cups flour.
* Kneed the dough for five minutes or so.  Maybe you have a stand mixer.  I use a wooden spoon and keep the dough in the bowl.  I don't kneed by hand because it takes extra flour, makes a mess and throws off the ingredient ratio.
* Let the dough sit in a loosely covered bowl for about 8 hours (like while you're at work).  If you're around, use a silicone spatula to gently fold the sides of the dough onto the top after a few hours.
* When you're done letting the dough rise (it should have at least doubled in size), gently fold the sides up, shape into a loaf, and put it with the "seam" side down on a baking stone or sheet.  There are videos online showing how to do this far better than I will describe here.
* Put your loaf in the oven. Bake at 400 F for 30 minutes.  I spray a little oil on the loaf to brown the crust.  When it's done, it should sound hollow when you tap it, and crackle a little when you take it out of the oven.
* Let it cool in the open for a couple hours.  If you put it in a container right away, steam will condense inside and make the crust soggy and increase the risk of mold.

Sourdough Pancakes:
These are DELICIOUS!  This will use up all the starter you have collected in the fridge over time.  They are extremely light and fluffy, and will be thin like crepes. This makes about 15 pancakes; good for 2 people for about $1 total and 450 calories each.
* In a bowl, mix 2 Tbsp sugar, 4 Tbsp oil (or applesauce), 1 egg, 1/2 tsp salt
* Mix in 2 cups of starter
* Get your griddle up to 350 F
* In a cup, mix 1 tsp baking soda and 1 Tbsp warm water, then mix it into the rest and wait a minute
* Pour 1/4 scoops of batter onto the griddle.  They will spread a lot because the batter is thinner than for regular pancakes.  When bubbles stop rising, flip the pancakes.  They'll take maybe 2 minutes on a side.

These are extra good with 1 mashed banana and 1/4 cup chocolate chips added to the batter.