1.   At its base, lots of fuel vapor is diffusing out.

2.   A static spark too weak to see in daylight can be strong enough to ignite a mixture of fuel vapor and air in the ideal concentrations.

3.   Both agree that volatile fuel vapors in the center tank of the Jumbo jet apparently exploded.

4.   But investigation has not turned up the source of the spark that set the fuel vapors afire.

5.   But the higher you go in the flame, the more fuel vapor has been used up.

6.   By displacing air in the tanks with an inert gas, fuel vapors could not combine with air in the tank to form a potentially volatile mixture.

7.   Efforts to start the mission Thursday were scrubbed when a cloud of highly flammable hydrogen fuel vapors leaked from a vent line.

8.   For months, the leading theory among NTSB officials in charge of the investigation has been that a spark of static electricity triggered the explosion of fuel vapors.

9.   Fuel vapors in the tanks could be ignited by metal-to-metal contact of pump parts, the federal agency said.

10.   Granito said that Boeing and the industry should have conceded years earlier that the buildup of fuel vapor in center fuel tanks in jets can create an unacceptable hazard.