Not when you fill them with explosive Acetylene gas. (Who does that?) On the way to a Super Bowl party, a 46-year-old Wyoming man didn't count on the forces of static electricity. When the balloon he had filled with welding gas rubbed across his back seats, it ignited and exploded — leaving him and his passenger with shrapnel wounds, broken eardrums, and potentionally two to six years in prison for "possession, use, or removal of explosives of incendiary devices."
Acetylene filled balloon blows up inside car [BoingBoing]