From Richard Gwilt:

In the early 70s, on a cello centre spring course, Mrs Cowan taught me viola. It went like this. We were working on the Mozart D-major Divertimento (K 136). On the morning of the concert she decided there were too many first violins, and not enough violas. “Boy”, she called [could never remember my name!]; “you must play viola tonight. Here is a viola. This ...”, she said, pointing to the second string down; “... is the D-string. This ...”; now pointing to the second space down on the stave; “... is a D.”  That was it. I learned viola that day, and played viola in the concert that evening!
(... what I find really extraordinary, is that just after writing that paragraph, I was searching through some ancient cassette tapes, and discovered, for heavens sakes - a partial recording of that very concert. Here's some of the Mozart, my viola debut, with Mrs Cowan announcing. Just remember, all those who are not lost down memory lane on hearing this, that this was the spring course, for the juniors. The summer course was for the seniors, the ones who could already actually play!)