Grand old duke of York

Sing (or teach):

Repeat, but with no consonants, so just one legato line of vowels.

Repeat, but switch around the words 'up' and 'down' ("And when they were only halfway down they were neither down nor up").

Repeat again, but this time omit the words 'up' and 'down' completely. Be entertained when people accidentally sing in the gaps.

Divide into two groups. Group A sings sings everything except 'up' and 'down'; group B inserts only those words in the gaps. Work on making it sound smooth.

Do the same again, except that the roles of Group A and B swap over every half-line. That is, the two groups take it in turn to sing (1) all the words or (2) just 'up' and 'down', switching every 4 beats or so. (Think about it: it's easier to demonstrate than to describe, but it gets weird in the second half of the song.)

With the right singers, this is also a good song to finish by inviting them to improvise harmonies.