You have probably performed a sandwich assay before, even if you didn't know it. Sandwich assays are the molecular sensing technology behind lateral flow tests like the at-home COVID test. After a sample is dripped onto the test strip, it combines with detector antibodies attached to tiny gold particles, which bind to any viral proteins present. When the sample reaches the test line, a second antibody anchored to the strip binds to a different site on the viral protein, creating a molecular "sandwich." As the gold particles accumulate, the test line turns dark red, becoming visible to the naked eye.