after opening yes
they don't have to, but they keep longer if they do
Yes, if you want them to keep for more than a day.
Mold and other discusting things on your food if you dont keep refriderated and cooked.
No, mayonnaise is a condiment.
No, mayonnaise is not a gas. Mayonnaise is a semi-solid or thick liquid.
It depends on the mayonnaise. You can even get egg-free mayonnaise which is equivalent to an infinite amount of mayonnaise for each egg.
Usually they are about the same.
mayonnaise
Mayonnaise is a noun.
Possibly...
No. There are hair products that that use the word "mayonnaise" in them but it is not the mayonnaise you eat.
Mayonnaise and mayo are the same thing. "Mayo" is just a shortened version of "mayonnaise."