How could a man use potatoes to masturbate?

Please don't be mean, I have a mentally retarded (FragileX) relative (50) who does this. We always found potatoes under his bed, and could never figure out why he took them from the pantry to his bedroom. Now he does live in a facility, but when he came home on a visit, this always happened. Then the facility called us after one home visit, and explained why he did this, and that when he returned after a home visit, he usually got an infection of some sort from the potatoes. They asked that we put a basket or something of clean potatoes in his room when he came home.

The potatoes were never damaged (well...um) or broken in any way, so we can't figure this one out. Even the men in the family claim ignorance.