Youko's birthday was on September 3rd.
Aki's birthday is on September 9th.

It is funny that Youko use a water gun looking like a M16

It would be actually quite an engineering feat to make a water gun capable of shooting accurately that far. It's much easier to make a solid projectile, such as those "gel blaster" beads, go that distance.

Just ain't fair

That's the White Death's descendant for ya