how do reptiles keep warm?

Most of them bask in the sun. Some of them will also take advantage of warm surfaces, such as rocks, but usually the warmth of the surface also ultimately comes from the sun.

Any external source of heat will do.

When they get warm enough, they find shade. They're surprisingly adept at keeping their body temperature more or less constant during the daytime.

Reptiles are ectothermic ("cold-blooded"), meaning they cannot regulate their body temperature internally as mammals and birds can. Instead, they have to rely on the external environment regulate their temperature. They do this by basking in the sun in order to warm up, and by retreating to the shade, a burrow or the water to cool down.

Unless the tub you have is extremely cheap (i.e. One of those disposable gladware deals), you can use an undertank heater. I have quite a few set up that way. You will probably want to get some type of rheostat (dimmer), as it won't get hot enough to melt the plastic, but it could get hot enough to overheat the gecko. I use plug-in lamp dimmers from home depot. Just make sure that the rheostat/dimmer can handle the number of watts the UTH puts out. (the one I buy at home depot handles up to 300 watts, which its far more than any UTH I've seen.)

What Do Reptiles Do

they lay in the sun most of the day and crawl into holes at night to keep warm becuse reptiles can't move when there cold

They have to sit in the sun for hours.

They can do very little until they warm up as they are cold blooded and cannot heat themselves as we can.

they lay on hot rocks that are in the sun so they get the heat form the sun on them and the heat from the rock on them too it is called sunning themselves