Lice don’t have lungs like other mammals, instead they breathe through small openings along their abdomens. Lice have an ability to close these openings up and shutter their breathing for about 20 minutes before they suffocate.

So, to kill head lice you have to force them to close their openings up for MORE than 20 minutes, at which point they will suffocate and die. 

To force them to shutter their breathing you need to coat the louse (and obviously in doing so, coat the hair) in something thick and slimy that won’t wash off. It almost doesn’t matter what it is. 

You can use cheap conditioner, even mayonnaise to kill lice - you must just make sure you’ve got all the hair coated and you leave the coating on for half an hour. If you’ve done it right it should be a one-and-done treatment. All the lice should suffocate and should then just fall or brush out.

Obviously food products like mayonnaise are going to leave a nasty smell/greasy aftertaste so they’re not ideal - but definitely better than nothing.

The best product is a dimethicone-based nit lotion that is oily and odourless. It is proven to kill nits effectively and is easy to work with. Look for nit lotions with 4% dimethicone.

Because water doesn’t stay on the hair for long you can’t drown nits in water (unless you can hold your breath for half an hour and stay underwater!!)

Like water, vinegar doesn't kill adult lice but it does deserve a mention here as it can help in the removal of nits (lice eggs). The acidic makeup of vinegar breaks down the glue-like substance that adheres the nits to the hair shaft and makes them easier to remove. 

If you plan to give vinegar a go, mix 50 millilitres of vinegar with 50 millilitres of water and use as a rinse. Once you’ve rinsed it through you can start picking the nits out with your fingers.