How do those people do those stunts by putting grenades under a warthog..

You probably aren't hitting the grenades in the right spot or shooting to the side.

The stunt is only done on the multiplayer Co-op, and I will tell you exactly how. First of all, you want to be on a level with a warthog, for example, The Slilent Cartographer. (or you could launch a ghost up in to the air, but it's not nearly as interesting) Get 4 frag grenades, and 4 Plasma grenades, by any means neccesary, (even if that means killing your own marines for more frags) And go next to the warthog and have you partner kill you. Your grenades will drop on the floor by the warthog. When you respawn, kill your teammate and his grenades will drop also. That's already 16 grenades. Go get full on more grenades (you might have to progress through the level to get more grenades). Make sure you have 8 grenades each (four plasma and four fragmentation) and kill eachother again.

Now thats 32 grenades near the warthog. Now how you ignite it: Slowly approach the grenades and pick up only 1 (make sure your partner doesnt have any, and that you only have 1) Throw the grenade at the warthog and when it explodes it will start a chain reaction and the others will explode too, launching the warhog sky high.

For a greater experience, try getting every grenade in the level (there is approx. 72 grenades in the level on Legendary difficulty) and then blow it. It goes sooo high you cannot even see it in the air and it falls back down like 10 seconds later. On some occasions the warthog never came back down. Have fun!

Its also fun to have a teammate in the warthog when you blast it. Doesnt really make a difference, but it is still fun to kill him. If you have any marines left then put them in the car and launch it.