You can also bind it from the console. First, decide what kind of grenade you want to use. There are two types. Timed grenades are just a regular grenade that explodes after a certain amount of time. Primed grenades are ones which you can start the timer and throw it when you want. I think the timer is 3 seconds so make sure to throw it in time or you will end up eliminating yourself. Primed grenades can eliminate yourself if you're too close, but timed grenades do not. You cannot shoot while priming a grenade, so be careful in which situations you use them in.
To make a timed grenade bind, type in the console..
bind key tossgren timed
To make a primed grenade bind, type in the console..
bind key +tossgren
Another type of grenade is impact. An impact grenade will explode when it hits a surface. You can make an impact grenade bind by typing in the console..
bind key tossgren impact
Impact grenades are no longer used very much because many of the good players couldn't handle them, and whined until they were changed. They are still used on a few servers, such as the EV1 servers and a few cable internet servers.
