Continuity mistake: In the scene where Merrill, Graham and the kids are returning home from town, and Morgan is holding his baby monitor to listen to the aliens, his fingernails are very long. When he gets out and climbs on top of the car to get a better listen, his fingernails are very short. (00:29:55 - 00:32:15)

Signs (2002)
Plot summary
Directed by: M. Night Shyamalan
Starring: Mel Gibson, Joaquin Phoenix, Abigail Breslin, Rory Culkin
Graham Hess and his young son and daughter, Morgan and Bo, struggle to recover from the tragic death of Graham’s wife. A neighbor struck and killed her with his car when he fell asleep at the wheel. Graham’s younger brother, Merrill, has been staying with the family. Devastated and bitter, Graham, a reverend, believes God betrayed him, and he has abandoned his ministry.
Mysterious crop circles appear on the Hess' farm and soon the world over. Alien scouts are spotted, and before long, ships arrive from outer space. With TV and radio their only link to the outside world, the isolated Hess family watch in horror as the alien threat turns into a deadly close encounter. The hostile beings are galactic raiders who harvest planetary life forms. With invasion imminent, the family barricade themselves inside the house. As the aliens are breaking in, they retreat to the cellar.
Bo: There's a monster outside my room, can I have a glass of water?
Trivia: When the alien's hand comes out of the coal grate for Morgan, if you look closely you will see that the hand was already through the grate and the alien was using its colour changing abilities to hide. So if you look very closely you will that that the hand comes UP from the grate and not through it, and if you look VERY carefully you can see the outline of the hand even before Morgen says "What?"
Question: Do the individual aliens have cloaking abilities? It seemed at that the dogs were reacting to something that the people could not see. Were they smelling cloaked aliens? Were they going crazy to protect their human owners and that was misinterpreted as an attack on the people? I thought they are walking around at least invisible at times to do their work without detection. I may be wrong on that.
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Answer: The aliens were not cloaked, not invisible. They had a chameleon-like ability to change their appearance to match their background. Recall the alien's fingers turning plaid when he held the boy. They seem to be that green color when not camouflaged. The better senses of the dogs did allow them to detect the aliens. However, one of the effects of alien presence noted in the movie was a change in personality; that is what happened to the dogs.
scwilliam