Ultra-high sensitivity bubble levels are much more sensitive than a typical carpenter's level that can be purchased in a hardware center.  However, they work on the same basic principles.  A "spirit" - most likely alcohol - is sealed into a hollow glass cylinder with a slight barrel shape.  Tipping the cylinder causes the bubble to float to its highest position in the fluid thus changing its position on a scale to give a measurement of the "levelness".  These ultra-high sensitivity levels have a thermal instability associated with them that should be noted by those who rely on them in their work.  Slight temperature variations along the axis of the bubble level, which can be caused by warm breath or the light from a flashlight, can greatly influence these instruments.  The videos below show this effect.  The videos run at increased speed for convenience, but the stop watch in the videos shows the proper elapsed time.

FLASHLIGHT EFFECT - a flashlight is shined on the end of the bubble level.

WARM BREATH EFFECT - a small amount of breath is blown through a tube held near the bubble level.

HEAT ENGINE - a small heating wire wrapped around the bubble level and carrying the proper amount of current can actually set the bubble into oscillation, thus producing a simple heat engine.

For full details of our experiments and calculations, please see the following manuscript (pdf format):  UNEXPECTED INFLUENCES ON ULTRA-SENSITIVE BUBBLE LEVELS