Let me describe some miracles and tell me who has done it.
Light always travels through different mediums in such a way that it takes the least time in going from one point to the other. To do that light has to undergo refraction while changing medium. How does light work out the least path which can be confirmed using advanced calculus to be perfect.
As you may be aware that a matter has more density in solid state as compared to liquid state. Water is an exception in this. Ice is lighter than water unlike other substances like metals. As a result, ice floats on water. Had this not been the case, all marine life forms would have died as they would have been crushed under heavy ice sinking in sea water.
Had there been no ozone layer surrounding the earth which protects us from the fatal effects of ultraviolet light, life would have been impossible on the earth. Who took care of providing this protective atmosphere surrounding the earth.
Nature takes care of the new born for its food in the form of mother's milk. In fact, mother's milk also protects the baby from many ailments. Scientists have also recently found a secret of nature that umblical cord can solve many ailments of a person for his lifetime and now there are banks for preserving the umblical cord for future use to tackle diseases of the person.
There are many such miracles scattered all around us in nature. I am not listing them to prove the existence of God, but just as you are asking the believers a question on miracles of God, they too can ask you to explain the force behind these miracles of nature. If you fail to answer them, their reply is that these miracles are due to God. If you disagree, can you prove that it is not so?