If you still have an older toilet in your home chances are that the tank is not insulated, that it is "sweating" during the summer and that you have a towel laying on the floor underneath the tank to catch the condensation droplets of water.
"Sweating" is no longer a problem with newer tanks for a number of reasons:
- The tank is insulated.
- The tank holds a lot less cold water than the older tanks.
Also there is less of a 'sweating" problem, even with older toilets, if the home is air conditioned, if you use a dehumidifier, or if you live in a "dry" area of the country where the HR (relative humidity) is low.

WHAT IS THE CAUSE FOR "SWEATING"?
The sweating occurs when the humidity in the air condensates on the cold surface of the toilet tank. Warm air can hold more water than dry air. The warm air cools down when it comes in contact with the cold tank and it looses its ability to hold moisture which is then released, much like the condensation on a cold glass of water.

DOES INSULATION STOPS SWEATING?
No, not always. An insulated tank will control the sweating to a certain degree. There still could be a bit of a problem if the water is super cold due to heavy usage or if the water is coming from a well. The insulation only slows down the condensing process.

BOTTOM LINE
Although the decision to purchase a toilet with a "lined" or "unlined" tank is a personal one, I would recommend to replace the older large volume toilet with one that has an insulated tank and preferably a dual flush system.

TIP
Consider going to a toilet that uses no water in a tank. Pressure assist or compressed air toilets use very little water and there is no sweating and no double flushing!