As Muslims, we believe that miracles ended with the prophet and that the last miracles were with our prophet Mohammad. It is, however a common Christian belief that miracles continue to happen in our time.
A Christian friend once sent me a link with claims of a modern-day miracle, saying that some wounds appear on a girl's body every year with no scientific explanation.
Presumably, scientific knowledge is still unable to explain her case; this does not, however, mean that it is a miracle. There is a significant difference between this sort of incident (which may appear miraculous, but is likely just not scientifically understood yet) and the true miracles of the prophet. How, then, can we explain to the Christians that this sort of thing is not and cannot be a miracle, since miracles ended with our prophet?