Isn’t it cruel to make a donkey pull a carriage?
That’s a little like asking isn’t it cruel to take your dog for a walk. The answer is a very definite no, provided the harness and carriage are properly fitted / suited and that it is not over-done. Driving keeps the donkeys fit and healthy, and provides them with interest and mental stimulation. They are very intelligent creatures, and the day after a trip out in the carriage will let you know that they enjoyed it. Guildenstern will often come running when he sees the harness.
