Haha....
Anything also can as long as you know the limit. Which when it comes to humans, the best advise is to stick to what it's built for. Meaning a guitar amp for a electric guitar, a bass amp for a bass guitar and a PA system for vocals.
The thing here is voicing. Which means each kind of amp is built with voicing range to suit the purpose. Some can be used of a variety of instrument, while others solely for it's own purpose.
If you put a bass on a guitar amp, you won't have the low punchy sound required, and to compensate that you pump up the volume. Result, you could drive the amp over it's limit and damage it.
If you use a guitar on a bass amp, it could sound muddy and boomy, or it could sound alright clean. Not likely you can get many useful sound settings, as the amp normally does not have overdrive channel and reverb.
A PA can be used for vocals, bass (thru DI), acoustic guitar and keyboard... so quite versatile, as it's what we call a "Full Range" system. But when you plug an electric guitar direct to it, very diffficult to get the right tone. So normally we put a mic at the guitar amp to feed to the PA.
Cheers Softies :wink: