A ranch woman takes her three sons to the doctor for physicals for the first time in their lives.
The doctor examines the boys and tells the woman that they are healthy but she needs to give them iron supplements. She goes home and wonders exactly what iron supplements are. Finally, she goes to the hardware store and buys iron ball bearings (BB's) and mixes them into their food.
Several days later the youngest son comes to her and tells her that he is pissing BB's. She tells him that it is normal because she had put them in his food. Later the middle son comes to her and says that he is crapping BB's. Again, she says that it is ok.
That evening the eldest son comes in very upset. He says "Ma, you won't believe what happened". She says "I know, you're passing BB's". "No", he says. "I was out behind the barn jacking off and I shot the dog".