The theory is that 'normal dogs' begin the day with a certain number of theoretical 'spoons' in the case of Billy here probably at least 10 but with a fearful dog like Jock maybe only 3! If there are 2 very stressful events in that dog's day then he is down to just one spoon and if he looses that then we really are dealing with a situation that needs careful handling.
Sometimes owners overestimate the 'number of spoons' and once a dog has lost them all there could be some truly awful consequences . The mauling of children 'playing' with dogs can be because that child has been 'playing' with a dog who is so to speak at the end of its tether and unable to cope with more stress.
Owners should try to recognise signs that a dog is becoming more stressed
Looking away and not making eye contact
Standing very still
These are all signs that the dog is not happy and should be left alone and not forced into any interaction with people or other dogs.