Specifically, when an unbeliever asks “How did Noah fit all the animals on the ark?”
I would make it clear that Christians can adopt a variety of responses to the question.
To answer from a mathematical and scientific perspective, one might say something like this;
Woodmorappe in his book Noah’s Ark: A Feasibility Study calculated that the number of animals would have been less than 16,000, assuming that a biblical kind is roughly equivalent to the group of animals we call a genus today. However, if the biblical kind is equivalent to the ‘family’ grouping, then there would have only been 2,000 animals. Probably it was somewhere in between. The animals would have been easily housed in small enclosures because most animals are small, on average the size of a rabbit. Even large animals, such as the biggest dinosaurs, began their lives small. In selecting creatures to repopulate the earth, it would make more sense to choose those that were young and healthy, rather than the older, mature ones. And the size of the Ark? It was huge. It had a capacity of over 500 railroad stock cars, enough to carry more than 120,000 sheep. So there was plenty of room on the Ark for the animals, for their food and water, and for Noah and his family.
But this answer assumes that the flood is a worldwide flood, and some Christians who read and try to faithfully interpret the Bible see that Genesis 6 could be talking about a local flood (still a large and extensive deluge, but not one that covered the whole world). If that’s the case then Noah wouldn’t have needed to pack in all the animals. The kangaroos in Australia and the moas and kiwis in New Zealand would have been safe and only a couple hundred types be represented on the ark.
But even that answer assumes that the flood story actually happened in history. Some Christians see the story of Noah’s flood as a as something like a saga (a story told to teach something about God and human beings, and not teaching something of history). If that’s the case, there was never really an ark or flood, and the problem simply evaporates.
That response might raise other difficult questions, but the larger point I’d want an unbeliever to understand is that, for Christians, this is all somewhat academic – it doesn’t really matter all that much – though it may be a matter of interest and conversation among some believers. For an unbeliever though, it’s all quite unimportant. You see, Christianity doesn’t stand or fall on whether the events of Genesis 6 literally happened, nor on whether the flood was local or worldwide. It stands or falls on the person of Jesus and if his resurrection is actual history. Whether that event happened is of prime importance, and just how to interpret these Old Testament stories can be figured out later. The big issue is where you stand with God, which is kinda the point of the story of Noah; For just as God did in the days of Noah, he has provided us with a way of escape—an ark—in the person of Jesus Christ. In Jesus—who lived a sinless life, died in our place, and rose on the third day—we, like Noah, may find “favour in the eyes of the Lord.”