This is a hand grenade; There are many different types: smoke, stick, incendiary, flash-bangs and of course fragmentation grenades like mine you see in this image. Upon detonation it throws fast-moving fragments of steel around the place to disable or kill an enemy. Some detonate on impact and some on a timed fuse. To use simply hold the lever in, pull the pin and throw. Once the grenade is in the air the safety releases flying away from the grenade, the striker-releasing detonates the primer which ignites the fuse (delay element) and the main charge explodes after the set time sending the fragments on their way. It's a pretty elementary and basic device. (Warning: Do not drop grenade at your feet, throw into room, foxhole or at enemy.)
In war it has occurred that a member of the unit has fallen on a grenade to prevent he and his comrades all being killed by it and in so doing sacrificing his life. It's maybe somewhat easier done these days with most soldiers wearing armour of some sort in their tac-vests however I believe the result is the same on most occasions. Death. It's a brave sacrifice of course and one many people could never understand however in a military unit strong bonds are formed and of course training is angled towards mission-integrity/success and so soldiers do things a civilian would never conceive of doing. They sacrifice themselves for the greater good of the mission and their team. Brave, courageous and selfless.
Having said that, I have actually thrown myself on a grenade once or twice in my lifetime as well, figuratively speaking of course. You know, on occasions when I've hung my neck out there on behalf of a colleague or friend, for the greater good or in defence of the colleague or even a concept or situation I'm trying to save. I think we have all done it in some form. Parents do it all the time. They go without on their kids behalf and they put their own lives in second place to allow their kids the best opportunities. In short they sacrifice their own well-being, their development, enjoyment or even health sometimes for mission-success. Sure, it may not end in their death, but it's sacrifice, often lasting years and years, nonetheless.
I do not believe that reward comes without sacrifice; That promotion at the office, rock-hard abs, a hot beach body, winning the Boston Marathon, becoming an astronaut or nurse, giving birth, developing a lasting and loving relationship with a partner...It all takes sacrifice. (And other things of course.) Mostly we don't instinctively make that sacrifice like a Team-guy (Navy SEAL might do by falling on a grenade); No, we have to work at it, grind away, take that ownership and responsibility I'm always going on about. Life is hard, it requires effort and persistence to design and create it the way you want it to be and that comes with sacrifice. Is it worth it? Well, that's up to the individual to decide. It takes sacrifice and only the individual can choose to make it.
So, please don't go and find a live grenade to fall on but do go and find it within yourself to make the sacrifices required to design that ideal life of yours. Go and find the ownership, take the responsibility and make the effort to get after it because that shit won't come to you for free. Sure, you can plod along like the rest of the sheeple but why stay on the ground with the turkeys when you can soar with the eagles!
Get after your life like it's the only one you got!