Hold the magnifying glass tool over an ant for at least 5 seconds. The ant will catch on fire and start to level up.
Free XP
There's no need to spend gold on tools to torture your ants in order to earn levels. You can simply take the pipette, pick up all or some of your ants and drop them in the water to the left of the ant hole. This slowly damages them, and will produce many a free and easy level up for your ants. Be sure to pick them back up as soon as you see any of them start to turn blue with damage, as you don't want to kill them.