Halve pears lengthwise; remove cores. (A melon baller removes cores easily, but a small spoon works, too.) Place sugar on a plate.
Heat a large skillet over medium. Press cut side of each halved pear in sugar. Pressing the cut sides of pears in sugar before cooking helps them caramelize. Place cut side down in skillet (fit will be snug). Cook until beginning to brown, 7 to 8 minutes.
Add 1/2 cup water. Cover; simmer until pears are tender, 5 to 10 minutes (depending on ripeness), adding more water if sugar begins to burn.
If liquid in pan is thin, simmer until thickened to a saucelike consistency; if it is thick, add more water. Serve sauce over pears.