Matthew 5:33-37 KJV
33 Again, ye have heard that it hath been said by them of old time, Thou shalt not forswear thyself, but shalt perform unto the Lord thine oaths: 34 But I say unto you, Swear not at all; neither by heaven; for it is God's throne: 35 Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. 36 Neither shalt thou swear by thy head, because thou canst not make one hair white or black. 37 But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
We should be living our lives in such a manner that people do not have to wonder if what we are saying is truth or not. They should not have to ask us to "swear" that what we are about to say is truth. When we say "yes" it should always mean "yes" and when we say "no" it should always mean "no".
This causes me to consider sarcasm. The world accepts sarcasm and I have to say that I was a pretty sarcastic person for many years. The reasons why I am not as sarcastic as much escape me, but it still creeps up sometimes. These verses cause me to be more aware and encourage me to try hard to remove sarcasm from my lips.