Toddler tears?

My son cries as much as any other toddler, but the weird thing is he never cries with tears. You know the big alligator tears you see on so many kids when they fall down or throw a tantrum or someone looks at them the wrong way, not my son.

I was struck by this the other day when we were playing with a neighbor and her two year old and in the span of 10 seconds this little boy went from happy to crying with great big globs of tears sliding down his face. Now my son is perfectly capable of going from happy to sad in 10 seconds, but there are never tears.

I feel really lucky to have been spared this aspect of drama with my son. I know you would probably get used to it if your kid did it, but tears on a little guy make me so so sad. I think I have seen my son's eyes get teary once, one of the times he fell and really hurt himself, but I don't think I ever saw a teardrop slide down his cheeks.

So my question is... how common is this? I know most children do cry with tears, will my son eventually develop this trait or is it something he will learn? Do any other parents have young children that do not cry with tears. My son is 20 months now and still no tears.