Ask a sample of women to record the sexes of everyone in their sib-group (brothers and sisters) including themselves and then total the number of brothers and the number of sisters recorded by all participants. You should finish up with a biased sex-ratio - there should be more women. If you ask the reciprocal question of a group of men, there should be an excess of blokes. You may now dispute that premise - you may claim that there is no such effect. You may also try to figure out why there should be an effect such as I've described. But I should first ask you: what is the sex-ratio in normal human populations? RA Fisher did the math 80 years ago to show that in any population under the action of natural selection (that's ALL populations, Middle America, even Southern Baptists) the sex-ratio must be close to 1:1. The fact that the sex-ratio is 1:1 is not, from many surveys of students over the years, obvious to all thinking people, whatever about any explanation.
With me, the thought of a "good classroom exercise" is the deed. I've explained the concept to my Mon and Tue Yr1 Biology classes this week and got them to write up the data from their families onto the lab white-board: red for girls and blue for boys of course. The alternative was to make drawings of a number of preserved specimens of insects, so there was a queue for the pens and I got my data.
(10+28) = (23+14)
it is embarrassingly close.
So why does it work? It works much better now than 100 years ago in Ireland when families were a lot bigger. It worked perfectly black and white until very recently in China where couples were limited to one child. If you ask women in such a society to record the sex of all the children of their parents you'll get a sex ratio of 0:1; and if you ask men it will be 1:0. It works for larger families too but less grossly.
Q. What possibilities are there for sex-distribution in families of two?
A. Three: all boys, all girls and one-of-each.
If you ask women about their families they'll never reply that their family is all boys. If you ask enough of them, you get a sex-ratio of 2F:1M - because the three possibilities for families which include at least one woman are FF, FM and MF. one-of-each is twice as common as either all girls or all boys, because girl then boy and boy then girl are equally likely events. This last fact is also not obvious to all thinking people.
That is indeed a pretty cool classroom exercise, a bit counter-intuitive, a bit of real data and a bit of hypothesis to test - science in fact!