I believe that prayer should be used to thank and praise God, not to ask for things. If your prayer is one of thanks and praise, of course it works, since you accomplished what you intended, giving thanks and praise. Although I would not take everything in the Bible at face value, I say you are misquoting the biblical verses to try to mock Christians who may be reading your question.
In Matthew 7:6, right before the verse you quote, Jesus says:
Don't give that which is holy to the dogs, neither throw your pearls before the pigs, lest perhaps they trample them under their feet, and turn and tear you to pieces.
He continues with (Matthew 7:7):
Ask, and it will be given you. Seek, and you will find. Knock, and it will be opened for you.
Jesus is saying that if his followers ask for "that which is holy" they will receive it. He didn't say that free miracles would be provided for the amusement of any joker on Yahoo! Answers.
In Matthew 17, before the verse that you quoted, a story is being told about a man whose son was epileptic. This man brought his son to Jesus' disciples to see if they could cure him, and they could not. Jesus then heals the man. When the disciples ask Jesus why they were unable to heal him, he says to them (in Matthew 17:20)
"Because of your unbelief. For most certainly I tell you, if you have faith as a grain of mustard seed, you will tell this mountain,'Move from here to there,' and it will move; and nothing will be impossible for you."
He is telling the disciples that they do, indeed, have faith smaller than a grain of mustard seed and that for this reason they could not perform the miracle. Whether or not it is true that someone with faith the size of a mustard seed could really move a mountain, Jesus isn't promising that ANYBODY can do this. Clearly he didn't think that even the disciples had enough faith to do it.
A mustard seed may be a VERY tiny thing, but that's exactly the point Jesus is making. He is saying that everyone has so little faith. So if you really wanted to use this verse to disprove the efficacy of prayer, you would have to find someone that Jesus agreed had faith the size of a grain of mustard seed (otherwise find an objective way to measure faith and compare with the size of a grain of mustard seed). If THAT person prayed for a mountain to move and it didn't, then you'd have a point, but until you find such a person I would suggest you spend less time trying to mock religious people and more time thinking about your own personal answers to the important questions of life.