Sentence:  This definition [of "tool"] is not simple, but contains several elements. The tool must not be part of the animal's body (a beak is not a tool); the user must manipulate the tool in some way for it to realise its function; and, finally, a tool cannot be attached to the <WSD>substrate</WSD>. This is a fairly clear definition, but does seem to produce some rather arbitrary distinctions (Hansell 1987b). The spider "Dinopis", for example, makes a small web which it holds in its legs, thrusting it down on passing ants. This is a tool, but all other webs, however complex, are not since they are anchored to the <WSD>substrate</WSD>. The woodpecker finch ... that uses a fine stick held in the beak to extract insect prey from wood, is a tool user, but a shrike ... that impales an insect on a thorn still attached to the bush is not. 
Meaning of the word marked with <WSD> tags:  A surface on which an organism grows, or to which an organism or an item is attached.