Because Pharaoh refused to set the Israelites free, God decided to punish him, sending ten plagues on to Egypt. These included:
The Plague of Blood.
God ordered Aaron to touch the River Nile with his staff - and the waters were turned to blood. The fish in the river died and the Egyptians couldn't drink the foul water. In the picture, Egyptians are digging around the river for a spring. But all they found was a spring of blood.
The Plague of Boils
The pharaoh, his servants, the Egyptians and even their animals developed painful boils all over their bodies. There were even boils on the Pharaoh's feet! A physician attended the Pharaoh's court but he could not cure them.
The Plague of Hail
God rained hail, fire and thunder upon Egypt and the hail struck down all the crops in the fields and shattered every tree. In the picture, the hail stones (red and white dots) are falling on a tree, as a shepherd and his goats shelter underneath. Hail storms were unusual in arid regions such as Egypt and were therefore extremely frightening. Moses, on the right, pleads with God to stop the storm.
Plague of Locusts.
God told Moses to stretch out his hand over the land of Egypt to bring a plague of locusts. The locusts covered the face of the land and swallowed up every crop and all the fruits of the trees. Afterwards there was nothing green in the trees, and all the crops in the fields had been destroyed.
Plague of Darkness.
Moses stretched forth his hand toward heaven, and there was a thick darkness over the land of Egypt. This darkness was so total that the Egyptians had to feel their way around. Then Pharaoh offered to set the Israelites free, on condition that they let their sheep and cattle remain. But the Israelites needed their animals to sacrifice to god, so they rejected Pharaoh's condition.