Help Fading A Prim

Ok I have this script that I can touch to make it fade, and it works great, but I want to be able to touch it again to make it back to 0 transparency.
When it is 100% transparent, I might not be able to click it because I can't see it, so could I change it to voice command..show/hide. I prefer touch, but either way I don't know how to make it work.
Could anyone help me...thank you.

float alpha;

default
{
state_entry()
{
llSetAlpha(1.0, ALL_SIDES);
alpha = 1.0;
}

touch_start(integer total_number)
{
while(alpha > 0.0)
{
alpha = (alpha - 0.1);
llSetAlpha(alpha, ALL_SIDES);
llSleep(0.05);
}
}
}

You can make it respond to chat commands using a couple of things: the "llListen" function to set it up (call that in the state_entry event usually), and the "listen" event which actually receives the chat messages. Here's a very basic example:

state_entry()
{
llListen(0, "", NULL_KEY, "show");
}

listen(integer channel, string name, key id, string msg)
{
if (msg == "show") {
alpha = 1.0;
llSetAlpha(alpha, ALL_SIDES);
}
}

That will listen to anybody in chat range saying "show" on the public chat channel. You can change to a private channel, change it to only listen to the owner, and change "show" command to something else.

And for future reference here is how you do a touch switch:

float alpha;
integer switch = 1;

default {
    state_entry() {
        llSetAlpha(1.0, ALL_SIDES);
        alpha = 1.0;
    }

    touch_start(integer total_number) {
        if (switch) {
            while (alpha > 0.0) {
                alpha -= 0.1;
                llSetAlpha(alpha, ALL_SIDES);
                llSleep(0.05);
            }
        }
        else {
            while (alpha < 1.0) {
                alpha += 0.1;
                llSetAlpha(alpha, ALL_SIDES);
                llSleep(0.05);
            }
        }
        switch = !switch;
    }
}

Thank you very much for your replies, it works great