How to convert MKV files to MP4 in a fast, simple and free way?

It often happens that your downloaded videos are not playable on PC or portable devices. What a frustrating situation! More often than not, this problem might imply that your playable device is not compatible with original video format, which reminds you to do some extra conversion with professional converting software. Hence, you will be more likely to search online for reliable software to convert MKV to MP4. It is somewhat an act of dredging for a needle in the sea. Any good solutions?