Cold corn tortillas tear (and taste crummy). Three ways to reheat them: In the microwave (about 30 seconds for six tortillas), in a skillet (40 seconds on each side), or directly over a gas flame (quickly and cautiously!). Keep warm by wrapping in foil or a towel.
Bubbles help keep the batter light, so be gentle when you incorporate the liquids and don't overmix. A few flour lumps are okay.