I know how to pick a random cell from one column
=INDEX($A:$A,RANDBETWEEN(1,COUNTA($A:$A)),1)

but I need to pick a random cell from more than one column, I tried this:
=INDEX($A:$F,RANDBETWEEN(1,COUNTA($A:$F)),1)
but it doesn't work.

I tried searching the other posts but I couldn't figure it out.
Please help, Thanks.