The House Judiciary Committee has questioned Google chief executive Sundar Pichai over why, when you search the word "idiot" in Google images, a picture of US President Donald Trump comes up.

"How does that happen? How does search work so that that would occur?" Democratic representative Zoe Lofgren asked.

Mr Pichai answered earnestly that Google stores billions of pages in its index, takes the keyword, matches it against the pages and then ranks them.

He said "things like relevance, freshness, popularity, how other people are using it" play a part in how items are ranked so that, at any given time, that rank will show the best results for the Google search.

"So it's not some little man sitting behind the curtain figuring out what we're going to show the users?" congresswoman Lofgren asked.

"[Instead it's a] compilation of what users are generating in trying to sort through that information?"

Mr Pichai answered: "Last year we saw over 3 trillion searches, so we don't manually intervene on any given search result."

Why was he even asked this?

In the past, Mr Trump has accused Google of rigging search results to suppress conservative viewpoints and highlight coverage from media that he says distribute "fake news".

In August, he tweeted:

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He even said this:

Sorry, this video has expired Trump accuses Google of search bias

Google has denied any such bias, and while the question has dogged tech companies for years, there's no evidence of an anti-conservative or any other political tilt.

While a statement from Google at the time said search results are not dictated by a political agenda but are generated by algorithms which are constantly being improved, Mr Pichai told Congress on Tuesday "we don't participate in partisan activities".

"We engage with both campaigns. We support and sponsor debates on both sides of the aisle."

Top committee Democrat Jerrold Nadler called the notion of bias a "delusion" and a "right-wing conspiracy theory".

He said Tuesday's hearing was the committee's fourth to address the topic — and he suggested he would move on to other topics, like the spread of misinformation online and Russians' efforts to influence US elections online.

What else did Mr Pichai tell the committee?

The issue of user privacy came up over and over.

Looming over the tech industry is the possibility of government regulation intended to protect people's data and a deeper look into whether gigantic companies like Google, Amazon and Facebook need to be broken up.

Asked for yes-or-no answers on what information the company collects, Mr Pichai demurred and attempted to convey that things are more complicated, with varying degrees of success.

The Google chief copped flak from the government for not attending a Senate panel in September. ( AP: Marcio Jose Sanchez, file )

Republican Ted Poe tried to pin him down on privacy.

He asked Mr Pichai, "I've got an iPhone … Can Google track me when I move?" (He then moved to the left toward his Democratic colleagues on the panel).

"Not by default," Mr Pichai answered.

When Mr Poe demanded a yes or no answer, Mr Pichai indicated it was complicated.

Mr Pichai also reiterated Google's position that it has no plans "right now" to re-enter China with a search engine generating censored results to comply with the demands of that country's communist government.

If that changes, Mr Pichai promised to be "fully transparent" about the move. He has said that he wants Google to be in China serving Chinese users.

Sorry, this video has expired Google CEO faces questions over plans for censored search engine in China.

Has he had to answer questions like this before?

Not in this setting. This was his first appearance before Congress.

It comes after he angered members of a Senate panel in September by declining their invitation to testify about election manipulation.

Mr Pichai's no-show at that hearing was marked by an empty chair for Google alongside the Facebook and Twitter executives who did appear. The committee declined Google's offer to send lower-level executives.

Mr Pichai went to Washington later in September to mend fences, meeting with some two dozen Republicans and indicating he also planned to meet with Democrats.

The former engineer took part in last week's White House meeting with other tech executives on getting government and businesses working more closely on accelerating emerging technologies such as artificial intelligence.

AP/ABC