What causes convection currents ( no links) just answers?

Answer 1

Both liquids and gases expand when heated, so their density becomes less. The less dense liquid/gas rises, and the cooler sinks.

Answer 2

Uneven heating causes differences in temperature which will result in differences in pressure of a fluid (gas or liquid). The less dense fluid rises creating a low pressure zone. Fluid from the higer pressure around the lower pressure move in to attempt to balance the pressure.

Answer 3

thermal gradients in a liquid or gas causing denser cold parts to fall and lighter warm parts to rise.