A level tool is a common toolbox addition used for making sure that all lines and edges are perfectly straight and horizontal. A level tool has a small capsule in the middle which is enclosed in a glass case to house a specific liquid. This can sometimes break, so it's important you know what liquid is in inside.
The most common form of liquid in a level tool will be ethanol. This will be an extremely diluted amount of alcohol. An incomplete measure of ethanol will be put into the glass capsule so that a bubble is created. This bubble will be used as the marker for measuring level lines.
Water is used as a liquid inside cheaper versions of level tools. The danger of using this liquid is the fact it cannot be used at all times. Most will leave the level tool in the garage which can become very cold. The water will therefore freeze and the level tool can become unusable.
Some manufacturers have started to sway away from using ethanol as a product in level tools. This is because of the danger it creates to humans if the glass capsule breaks. Other diluted spirits with alcoholic qualities are used as an alternative. These have substantially reduced ethanol usage in level tools.