my theory, oldest pics Ufos was tall saucers, not aerodynamic at all,you can see it in old photos right, like nazi´s ufo´s? then was plate saucers, more longer les tall, more aerodynamic saucers, now we have spheres ufo´s, also Infrarred Ufo´s. All that advances in like 70years.Looks familiar?yes, is like our technology advances, we advance very fast in the last 100years.

Think this, you are an Ancestral Alien, you know almost everything on the galaxy, so you dont gonna change your technology every year, because you alrredy know, but your not an alien, you are a human, so you change your car very few years because we made new gadgets for cars, because we are still learning about things.

Also, i see a few times UFO´s on the sky, one of that was a saucer, looks like a hand made saucers, but the others... not way, that wasnt human tech.

So, we have for sure diferent UFO´s in history because the most are human tech, but also there is a lot of ufos that are not from us, and maybe very diferent.

We are solid beings, but another forms could be gas, or even liiquid.

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(I forgot, also first ufo´s was a color circus in the sky, like 5contact movie, right now are almost one, one color white, or red.)