Sima (not to be confused with Zima, although I suspect there may be a connection) is a sparkling Finnish beverage made by briefly fermenting lemon and sugar.
Now why would we participate in such an obscure cultural tradition? Is it because we are honoring our Finnish ancestors? Or perhaps we were taught the technique by an elderly Finnish neighbor? Nope, nothing as cool as that. Mom got it from a Time Life book. Although to be fair it is from the 1969 Food of the World series which in my humble opinion is fairly badass. I recommend grabbing them if you come across them, especially if you find not only the cookbook but the larger book it came inside of.
Back to the wonders of sima: not only does it fulfill my love of all things sparkly beverage related it also avoids the guilt of buying can after can of seltzer (which is essentially using fossil fuels to ship water).
Here is how it is made: you take a few lemons and peel them and then get rid of the pith (yucky white stuff).

(The batch pictured included ginger which is what is being chopped.)
Then throw the peel and lemon into water with some white and brown sugar. You heat this up to dissolve the sugar. You then let it cool before adding yeast (so the yeast isn't killed) and then let it sit at room temperature for 12 hours. This is the first fermentation.

Then you take sanitized bottles (we use old Grolsch lager swing top bottles, I'm frankly not sure what other kinds would work) and add sugar and two or three raisins before pouring in the liquid.

If you figure out how to do the pouring step without making a total mess feel free to enlighten me.

You then seal them and let them sit at room temperature until a mystical process takes place which results in the raisins rising to the top (some mumbo jumbo about carbon dioxide...I don't believe a word of it). See the raisin at the top!?
Then you chill and it's ready to drink! Now you may wonder, with all that fermentation, wouldn't it possibly be alcoholic? According to my mom the short fermentation and small amount of sugar aren't enough for it to get very alcoholic (probably less than half a percent).

But a word of caution: open these with all the care you would use for a bottle of champagne! There have been a few explosive sima incidents and I wouldn't want anyone to loose an eye.
Now to totally disregard tradition here are some of our variations: we tried adding some ginger with the lemon once, it was good but not really worth the effort, it wasn't nearly as gingery as we expected. We also added lime once which I liked. Lemon lime soda!
And a note on peel: mom will only use the peel of the lemon if we get organic ones (pesticides and all) so if that isn't an option you can always leave the peel off. The sima will come out good but without the hint of bitterness the peel gives it (which may or may not be a good thing in your opinion).
Here are our instructions based on the Time Life ones:
Sima variations
makes 9 - 10 pints
3 small lemons
a chunk of ginger
or
2 small lemons
1 lime
or
a couple large lemons
If fruit is organic separate zest so you can remove as much of the white pith as possible. Slice peel and fruit thinly.
Peel ginger and grate or chop.
Bring 5 quarts of water to a boil. Add fruit and 1/2 cup each white and brown sugar.
Let cool to barely warm (100 to 105 F/40 C) on wrist.
Add 1/8 tsp. (baking) yeast
Leave at room temperature for about 12 hours.
To bottle in pints:
Wash the bottles in hot, soapy water. Rinse very well and drain.
Add 2 raisins to each bottle. Using a funnel, add 1/2 tsp. sugar to each bottle.
Then fill with sima, straining out fruit, and cap.
Leave at room temperature until the raisins rise to the top. Check frequently because
the raisins may subsequently fall but the bottles are still ready to refrigerate. Keep
cold until serving time (or else!) and open cautiously.