Underwater waterfalls: upward and downward flow

How can you make a waterfall underwater?

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion.


  • large con­tain­er of wa­ter;
  • two emp­ty bot­tles;
  • red and blue food col­or­ing;
  • two rub­ber stop­pers;
  • dou­ble-sid­ed tape;
  • two emp­ty glass­es.

Step-by-step in­struc­tions

Fill a bot­tle with hot wa­ter and tint the wa­ter red. Fill a sec­ond bot­tle with cold wa­ter and tint the wa­ter blue. Сlose the bot­tles with rub­ber stop­pers and use dou­ble-sid­ed tape to fix each bot­tle in a glass. Care­ful­ly place the glass­es in a con­tain­er of wa­ter and care­ful­ly re­move the stop­pers from the bot­tles. Ob­serve as the cold (blue) wa­ter sinks and the hot (red) wa­ter ris­es!

Process de­scrip­tion

If a sol­id body is less dense than wa­ter, buoy­ant force will cause it to float on the wa­ter’s sur­face. This is true for liq­uids as well! Wa­ter does not have a sin­gle, con­stant den­si­ty – over a wide range of tem­per­a­tures, the warmer wa­ter is, the low­er its den­si­ty, and vice ver­sa. There­fore, hot wa­ter ris­es, and cold wa­ter sinks. The phe­nom­e­non when streams form due to tem­per­a­ture dif­fer­ences is called con­vec­tion.

A sim­i­lar ex­per­i­ment is in­clud­ed in the MEL Physics sub­scrip­tion!