6. (6 points) Fill in a natural number in each of the $\square$ and $\triangle$ to make the equation true.
$$
\square^{2}+12=\triangle^{2} \text {, then: } \square+\triangle=6 \text {. }
$$
【Solution】Solution: According to ${ }^{2}+12=\triangle^{2}$, we get $\triangle^{2}-\square^{2}=12$, so $($ 口 $+\triangle)(\triangle-\square)=12$;
(1) When 口 $+\triangle=12, \triangle-\square=1$, we solve to get $\triangle=6.5, \square=5.5$,

Since $6.5, 5.5$ are not natural numbers,
it does not meet the requirements of the problem;
(2) When 口 $+\triangle=6, \triangle-\square=2$, 
we solve to get $\triangle=4$, 口 $=2$,
at this time 口 $+\triangle=2+4=6$;
(3) When 口 $+\triangle=4, \triangle-\square=3$, 
we solve to get $\triangle=3.5, \square=0.5$,
Since $3.5, 0.5$ are not natural numbers,
it does not meet the requirements of the problem;
In summary, we get
When $\triangle=4, \square=2$,
$\square+\triangle=2+4=6$.
Therefore, the answer is: 6.