I expect many people know this one, but it's a useful teaching aid when understanding the relationship between arrays and pointers

  int array[10];
  *(array+1) = 56;
  array[2] = 4;
  3[array] = 27;

The first two are obvious, but the third is also legal. It works because array indexing is just sugar for pointer arithmetic, so array[2]=4 is identical in meaning to *(array+2)=4. Therefore 3[array]=27 is identical to *(3+array)=27 and so is legal. But just because you can doesn't mean you should.