Dad & Dave are crossing the desert with their horse

It’s 100 degree heat and they’ve run out of water when they come across an oasis. They run and jump in and drink and cool off when they realise their horse is still standing there and making no effort to jump in and drink. Dad says let’s lead him to the water but he still doesn’t drink, finally Dad says to Dave, you hold the horses head under the water and I’ll suck from the back using straw action and then he should drink. Dave holds the horses head under the water and Dad starts sucking.

After about a minute Dad yells out “Dave, hold the horses head up a bit he’s sucking mud”!