[I]t was followed by a long scream of rage mixed with a roar of complaint: “AAaargwannawannaaaagongongonaargggaaaaBLOON!” which is the traditional sound of a very small child learning that with balloons, as with life itself, it is important to know when not to let go of the string. The whole point of balloons is to teach small children this.

Terry Pratchett (1948-2015) English author

A Hat Full of Sky, ch. 11 “Arthur” (2004)
    (Source)

This is apparently the origin of the much more frequently found paraphrase:

There are times in life when people must know when not to let go. Balloons are designed to teach small children this.

This shorter form has been used, among other places, in a Change.org petition to Death to reinstate Pratchett after the author's passing. It is possible Pratchett may have used it somewhere else, but I am unable to find it (it does not show up in the alt.fan.pratchett board in any message from him).

See also Cox.