Mayonnaise, the hydrogen of condiments, is pretty freaky stuff. It's an emulsion, one liquid suspended in another. It's basically a cup of oil suspend as tiny droplets in a few tablespoons of lemon juice and egg. That sounds backwards, but lecithin is a powerful emulsifying agent. Mustard helps emulsify, too.
Commercial mayonnaise is pretty good but homemade is supposed to be easy and better. I tried last night with a simple recipe and it didn't go so well. The first attempt was going perfectly up until I added the last bit of oil at which point the sauce broke. You go from having a silky suspension of oil in egg to chunky, greasy mess of egg suspended in oil. The second attempt was better but I got the flavouring wrong. All told it took me an hour. I can't imagine what it'd be like with a hand whisk.