searching with part of a word

When searching for a word, sometimes I get this word as wel as longer words where the word given is part of. E.g. when asked for sphider I also find sphidering. However, when I type in an incomplete word (because I'm not always sure about spelling variations), I never get things like that. Is that because only indexed words are sought after, or is there another reason?
Jan

Word Stemming is an option in Sphider.

Admin > Settings > Use word stemming (e.g. find sites containing "runs" and "running" when searching for "run". Should be enabled before indexing.

Anytime you change that setting you need to re-index any sites/pages.

You can only get results for words that are indexed. So if a word is not indexed, no part of it can be found.