From: Mark 8:11-13
The Leaven of the Pharisees
 The Pharisees came and began to argue with Him (Jesus), seeking from Him a sign from Heaven, to test Him.  And He sighed deeply in His spirit, and said, "Why does this generation seek a sign? Truly, I say to you, no sign shall be given to this generation."  And He left them, and getting into the boat again He departed to the other side.
11-12. Jesus expresses the deep sadness He feels at the hardheartedness of the Pharisees: they remain blind and unbelieving despite the light shining around them and the wonderful things Christ is doing. If someone rejects the miracles God has offered him, it is useless for him to demand new signs, because he asks for them not because he is sincerely seeking the truth but out of ill will: he is trying to tempt God (cf. Luke 16:27-31). Requiring new miracles before one will believe, not accepting those already performed in the history of salvation, amounts to asking God to account for Himself before a human tribunal (cf. Romans 2:1-11). Unfortunately, many people do act like this. But God can only be found if we have an open and humble attitude to Him. "I have no need of miracles: there are more than enough for me in the Gospel. But I do need to see you fulfilling your duty and responding to grace" ([St] J. Escriva, "The Way", 362).
12. The generation to which Jesus refers does not include all the people of His time, but only the Pharisees and their followers (cf. Mark 8:38; 9:19; Matthew 11:16), who do not want to see in Jesus' miracles the sign and guarantee of His messianic mission and dignity: they even attribute His miracles to Satan (Matthew 12:28).
If they do not accept the signs offered to them, they will be given no other sign of the spectacular kind they seek, for the Kingdom of God does not come noisily (Luke 17:20-21) and even if it did they in their twisted way would manage to misinterpret the event (Luke 16:31). According to Matthew 12:38-42 and Luke 11:29-32, they are offered yet another sign--the miracle of Jonah, the sign of the death and resurrection of Christ; but not even this remarkable proof will lead the Pharisees to shed their pride.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.