not without a permit
Assuming you are meaning some kind of a travel trailer, the laws would depend on the locality. In many states in the USA, it is legal to build one's own utility trailer, and presumably travel trailers. Licensing requirements would depend on the weight of the trailer, or the gross weight of the trailer and cargo.
There is no legal way to do this without properly installing a UL listed transfer switch. Transfer switches have a mechanical interlock that prevents the utility power and generator power from being connected at the same time. The codes and regulations protect you from having your generator and wiring destroyed, but it also protects the utility workers from injury. The problem is, if a utility worker were to come out and install a meter, not knowing there was a generator hooked up. If there were a power failure, say, because of downed line, and the utility company sent workers out to work on it, they would think the line was dead. Your generator could be backfeeding voltage onto the lines and could conceivably electrocute the worker. There are actual cases of people killed under these exact circumstances. If you need temporary power during remodeling or construction, there are ways to get power, but you need to hire a licensed electrician to do it right and be legal. If you need power because your utility disconnected for nonpayment, they will take a dim view of any jury-rigged arrangement that could jeopardize the safety of their workers. They will take legal action against you, guaranteed. Better to run a separate cord around the house to power the lights and remain isolated from the buildings wiring.
legal requirement
Cloning of animals is legal, but cloning of humans is considered to be a taboo and is not legal.
NO
It is legal to build a nuclear power plant in the U. S., provided the licenses and permits are granted.
are gas powered bicycles legal in California see this doc; http://bit.ly/cZx9kz
yes, the president wants to build more because it uses cleaner energy so we do not run out of our other resources
Oy! He wants to install a nuclear reactor in his basement. What is the world coming to? (Pardon me. I've been watching Fiddler on the Roof. And yes I know a RTG ain't exactly a reactor. But still...) No, that would not be legal.
yes it is very legal iam from there
No. It is illegal. Wii points are to be used only from Wii points cards. Any other way of getting them (as by the Wii points generator) is not legal.
There is no free and legal membership generator. Club Penguin is owned by Disney.
Nuclear bombs are illegal since bombs could potentially be harmful to others and places, so no it is not legal.
For the countries that have them, yes.
Yes it is leagal and completely worth it.
Do you have one for sale?
No. It is not illegal to build a nuclear device any more than it is illegal to say mix up ANFO or make a lockpick, provided you have the required permission to possess the materials and technology need for the job. Simple creation and possession of something is not usually illegal, regardless of its possible uses, and nuclear devices are no different.