How do we change the way print displays a table

Assuming I have a piece of code such as the following

aTable = {aValue=1}
aTable_mt = {}
print(aTable)  

What must I do to make Lua print something like aTable current aValue = 1 as opposed to table: 0x01ab1d2.

So far I've tried setting the __tostring metamethod but that doesn't seem to be invoked by print. Is there some metamethod I've been missing or does the answer have nothing to do with metamethods?

__tostring works:

aTable = {aValue=1}
local mt = {__tostring = function(t) 
                           local result = ''
                           for k, v in pairs(t) do
                             result = result .. tostring(k) .. ' ' .. tostring(v) .. ''
                           end
                           return result
                         end}      

setmetatable(aTable, mt)    

print(aTable) 

This prints aValue 1 (with one extra whitespace, remove it in real code). The aTable part is not available, because aTable is a variable that references the table, not the content of the table itself.