A butterfly drinks by uncoiling its proboscis (tubular tongue) and inserting it in the crevices of the mulch or along the edge of a water droplet drawing water through its tube-shaped tongue. The smallest amount of moisture, or droplet of water, can provide a good drink for a thirsty butterfly.
Butterfly's moisture requirements are usually met in the form of raindrops, water droplets, moist soils, moistened mulches, and animal droppings. Routine watering of the butterfly garden is generally more than adequate to fulfill their moisture needs. Watering the flowerbeds on hot days will usually result in several butterflies landing to drink from the moist or mulch.
As unlikely as it sounds, butterflies are also attracted to misting, or spraying, water. I use a bird mister regularly to water my landscape because it puts a fine mist rather than heavy spray, and within minutes of turning it on butterflies show up along with the many songbirds.
Butterflies can often be seen drinking, or pudding, from mud puddles and areas of concentrated livestock urine in order to obtain required salts and minerals as well as water. They also procure moisture, and minerals, from fresh manure such as bird droppings, horse, and cow manure. Butterflies certainly know where to find needed nutrients.