Buy some key limes (10 cents each at my local grocery store), and a lime squeezer. A lime squeezer is a hand-operated metal gizmo that squeezes limes and filters out the seeds. (Resist the urge to buy a fancy electric citrus juicer or an internet-connected, GPS-enabled lime squeezer. Squeezing the lime by hand is an important part of the experience.)
Cut a lime in half and squeeze each half into a glass. Then fill the rest of the glass with regular or bubbly mineral water, and stir.
This really tastes great! After getting accustomed to this drink, I find that artificially flavored, corn-syrup-laden lemon/lime soft drinks taste terrible by comparison. And I look forward to the enjoyable little details of making one: slicing the lime, the satisfying "squoosh" of squeezing it, the scent of fresh citrus . . . like making tea, it is a fun little ritual.