Challenges include the type of paint used to paint outdoors, animals, bugs, onlookers, and environmental conditions such as weather. Acrylic paint may harden and dry quickly in warm, sunny weather and it cannot be reused. On the opposite side of the spectrum is the challenge of painting in moist or damp conditions with precipitation. The advent of "plein air" painting predated the invention of acrylics. The traditional and well-established method of painting "en plein air" incorporates the use of oil paint.