[header] How to get evidence thrown out in court [title] Challenge a witness's competency. [step] A witness is only competent to testify about an event if he has personal knowledge of it. Object to any witness who begins testifying about an event without first establishing that he observed it. 
[substeps] As the witness is testifying, you or your lawyer stands up and says, " objection, your honor " and then states your grounds. Here, the grounds are " lack of personal knowledge.