Possibly in Bad Taste but......
.... I was never noted for good taste in jokes!
The family wheeled Grandma out on the lawn in her wheel chair where the activities for her 100th birthday were taking place.
Grandma couldn't talk very well but she could write notes fairly good when she needed to communicate.
After a short time out on the lawn, Grandma started leaning off to the right and some family member grabbed her, straightened her up, and stuffed pillows on her right.
A short time later she started leaning off to her left, and again, a family grabbed her and stuffed pillows on her left.
Soon she started leaning forward, and a family member again grabbed her and tied a pillow case around her waist to hold her up.
A nephew who arrived late came running up to Grandma and said, "Hi Grandma, you're looking good, how are they treating you?"
Grandma took out her little note pad and slowly wrote a note to the nephew, "They won't let me fart".