It seems that creationist use "the flood" to explain away a lot of things which would prove the Earth is billions of years old. For example the grand canyon took millions of years to form, so creationist say, "uh oh..that fucks up our idea the Earth is 6000 years old...well when the flood happened the water hit the grand canyon at the speed of sound forming it in 5 minutes...science averted". Or when you point out the various layers in which only specific fossils are found (ie there are no bunnies found in the Devonian era), they explain it away like "well when the flood happened certain animals were able to make it to higher ground so that explains the layers". You get the picture, they always find convenient ways of explaining why science is wrong because "the flood" did it. What happened to the dinosaurs? Flood. What caused the ice caps to form? Flood. What caused the craters on the moon? (Im fucking serious they explain the craters on the moon with the flood) Flood. So what do you say to people like that? Well you take them through an exhaustive list of reasons why the flood never happened or could have happened especially the way their mythology tells it.
According to the Bible the ark was 450 feet x 75 feet x 45 feet. For a frame of reference the Titanic was 882 feet x 92 feet x 175 feet. So the "ark" was half the size of the titanic which fit a couple thousand people.
Now that the size of the ark has been established take them through what needed to fit on the ark. Noah and his family, over 17 million species of animals, enough food to feed all of the animals, and enough water for them to drink.
No doubt the last argument will be met with "Noah only brought distinct kinds" which still leaves you at about 12 million animals. Also "he brought animals that breath with their nostrils". So whales made it on the ark? But insects they were left in the water I guess since they don't have nostrils? Not that it matters because all the fish would have died with such a crazy change in sea level and temperature and salinity.
Then ask them who cared for the animals. The zoo in DC has 10 times more employees to care for a few hundred animals, so how did less than 10 people care for millions of animals?
Then tell them, "it doesn't really matter though because all the animals and Noah would have froze to death when the 'water covered the highest mountain'".
Then ask where all the water went? You cannot destroy matter, that water had to have gone somewhere. Did the water magically dry up and not form clouds which would simply rain the water back down to Earth?
Then ignoring the space issue and the freezing to death issue ask them what happened when the ark landed. Since the Earth was covered by water all plant life is now dead, so what do the herbivores eat? More importantly how do the carnivores eat with out causing a species to go extinct?
Also how do they explain the lack of genetic diversity? Apparently according to their story no species including our own would have a sufficient gene pool to reproduce. The cheetah has about 3000 members living in the wild, but is lacking sufficient genetic diversity for a healthy gene pool. So how did two members of a species survive?
After all of this if they still believe the flood ever happened they are retarded and they deserve their religion telling them what to do.