The presenter tunes his guitar, presses a key on his notebook computer (connected to a beamer), and starts to sing towards the audience:

To common good conduces
who never fails to give
his body fluids and juices
that someone else may live!

It doesn't hurt (or little)
and it won't make you sick.
You'll be surprized, but it'll
take just a little prick!

The act is recommended
to all and everyone.
A few will feel offended,
but most will find it fun!

When done, you may feel weary,
exhausted by the chore,
but once you've grasped the theory,
you'll like it more and more!

For best re-alization
use our laborat'ry
with closest observation
and test for HIV!

Now if your zeal is ample,
and you come without fear,
you'll find for good example
your photos shown up h-

... uh, uh, sorry, wrong slide show. I'll start again ...