In the moment when you begin to pray, the devil attacks you; and after the first words, after the first short prayers you make, he puts all kinds of unimportant, worldly thoughts in your mind. Even curiosity about the time of day or whether it is sunny or cloudy works in your mind. All of these appear as innocent behavior, but they disrupt the voice of prayer in our heart.
Father Paisios of Mount Athos says that these thoughts which appear in our mind at the time of prayer are like airplanes. First, you hear them from afar very faintly as a noise without much intensity, then the noise grows and grows, and when they arrive over your head, they overwhelm you with their noise and then go away. But if you enter into conversation with these thoughts, they will make your heart into an airport.
Father George Calciu, “Prayer is a Struggle,” Adrian and Mihaela Ulmer (trs), Father George Calciu: Interview, Homilies, and Talks, Saint Herman of Alaska Brotherhood, pp. 138-139.