Each piece, once attached, is now part of the toy's new body. Stitch a chainsaw to a toy and it now has control over a chainsaw. Add a gun, and now guns DO kill people. Well, toy guns, at least.
The one thing seemingly standing between us and certain destruction is that toys do not reveal themselves as alive to us humans. However, unlike the rules in Jim Henson's The Christmas Toy, the Toy Story universe's toys have no repercussions if they break the rules of never moving in front of humans, either accidentally or on purpose. Moving and conversing with animals is perfectly accepted, and during the end of the first Toy Story, Woody rallies all of Sid's toys to show themselves as very much alive. This means that the toys are bound not by a hard-and-fast rule but by the Honor Code, which is actually terrifying on multiple levels. However, before exploring why, we have to try to determine what, if anything, is giving the toys their sentience.