Use the Search tab in the card index on the left side of the screen to bring up a list of articles containing any word you wish to search for. All of the articles in the open book or books will be searched.
For example, to find all articles which mention electricity, open the books you wish to search, then click on the Search tab.
Place the cursor in the white box just below the Search tab by clicking with the mouse. Type electricity, and press enter on your keyboard. This will bring up a list of all the articles in the open books which contain that word, ordered by relevance to your search. The most relevant entries appear at the top of the list in red. The word will be highlighted in solid red every time it occurs in each of the articles.
You can also type in simple queries in plain English as your search, for example:
"How many bones in the human skeleton?"
"How do bees transmit information?"
The search will provide a list of entries most likely to answer your question.