Spook

Frosty is almost 30 years old and his eyesight is not very good. At dusk shadows frighten him and in every lesson he spooks at least once with his rider. His great age ensures that the furthest he moves is two steps to left or to right but it's often enough to unseat a child, and so far in three days of teaching I've had one child fall off and a couple of near misses. Children are starting to say: "Please don't make me ride Frosty!"
I have to repackage the pony.
So I'm going to explain to the children (and their parents) that the arena is an artifical environment. When they hack out dogs will bark at their ponies, plastic bags will flap in the wind, boy racers will roar past with loud tooting - and their ponies will spook.
To prepare the children for spooking they will each get a chance to ride Frosty. If they can sit Frosty spooking and it doesn't frighten them half to death then they're ready to hack.
If they fall off it'll be gentle for Frosty comes up only to my elbow.