Definition: Something that will cause an angry or violent reaction.
A red rag to a bull is something - perhaps a comment or a criticism - intended to deliberately provoke someone else into reacting negatively. The expression may appear to have originated from bull-fighting, but this is not the case. There are, apparently, a large number of things that were believed to be distracted or angered by the waving of a red cloth, and the original expression, in fact, mentioned pheasants. Soon after came vipers, and then turkeys, and even Frenchmen! It wasn't until 1873 that bulls were added to the list. Strangely enough, bulls are in fact colour-blind, and therefore the colour of the rag is irrelevant. Nevertheless, the expression remains!
"Don't tell him you support Manchester United - it's like a red rag to a bull!"
"It was like a red rag to a bull when Samantha heard John say that men should be paid more than women."
What to you is like a red rag to a bull? Write some example sentences.