flaps again

"Michael Ash" wrote

I recall an unfortunate incident in which a poorly-placed water bag had
somewhat reduced my right aileron authority, and I thought I could get
it
out of the way in the middle of the takeoff. The aircraft quickly
disabused me of this notion, and I recovered without breaking anything,
but it made for an embarrassing show. (Why is it that you can be
absolutely certain that someone is watching when you screw it up, and
absolutely certain that nobody was around to see it when you make a
greaser landing?

I'm not up on glider operations, so maybe you can clue me in.

What is a water bag? Ballasting? Why would it have been laid on the
ailerons? How or why could it be forgotten? How big is it?

Thanks for taking the time to fill in a "motor head!" g
--
Jim in NC