Try to explain that the sum of the product of four integers and 1 is a complete square number

Try to explain that the sum of the product of four integers and 1 is a complete square number

In[1]:= Expand[a (a + 1) (a + 2) (a + 3) + 1]
Out[1]= 1 + 6 a + 11 a^2 + 6 a^3 + a^4
In[2]:= Factor[%]
Out[2]= (1 + 3 a + a^2)^2