whats to use, _root or _global?

hello,

For most of my flash scripts, i have been using _root. for variables that being used by different scripts in different areas,

but i have recently noticed that _root. does not affect scripts in different scenes,

say that i have have _root.hello = 10; in scene 1, and I also have that in scene 2,
now if i change _root.hello in scene 1, my _root.hello in scene 2 will not change

I experimented on using _global but it doesnt work either,

is there a similar script to _root. that might be able to change variables in between scenes? (im using MX7)

thx a bunch!