I purchased a pair of those hand grip excercisers today, but after reading the warning, I’m afraid to use them. The paper card they were attached to warns:

Consult your doctor before starting any exercise program. Exercise programs of any kind present an inherent danger to the participant. Serious or fatal injury can occur. All equipment is intended to be used by adults and only in the manner shown/illustrated/described. Anyone under the age of 18 should have adult supervision. Always follow instructions (if included). Always use proper techniques and common sense when exercising. Always check your equipment thoroughly. (emphasis added)