One very easy way to make invisible ink is to use lemon juice. Dip a paint brush in lemon juice and use it to write your message. Once the lemon juice dries hold the paper up to a light bulb or use a hair dryer to heat up the paper. The lemon juice will turn brown revealing your message.

Choosing from typical household liquids, lemon juice or vinegar wouldbe good candidates for making invisible ink. After writing, you let the message dry and then make it visible by ironing. If you'd like to know more about invisible inks and how they work, you might want to look at one of the questions we recently answered:

I don't know if it is the best - but at least for here -- lemon juice works very well on rag, cotton and wood pulp papers -- use a candle or stove to 'develop' the writing.

Another good choice is one of the ultra-violet dyes -- e.g. Ultra Violet Products A800 is a common choice... Colorless, but blue emission under long-wave UV.

Finally, any colorless acid will effect a common indicator, so you can write with Sodium Bisulfate and develop with methelene blue, or even red cabbage dye (a nice red/green indicator of pH).